#tz package

Time zone support layered on top of core's NaiveDateTime. Import connect0459/almanac/tz for the TimeZone trait, Utc, FixedOffset, DateTime[Tz], and Location (IANA tzdata lookup by zone name, e.g. "America/New_York").

#Key types

Type	Description
TimeZone (trait)	Resolves a NaiveDateTime to a UTC offset; implemented by Utc, FixedOffset, FixedZone, and Location
MappedLocalTime[T]	The result of resolving a local (wall-clock) reading: Single, Ambiguous (DST fold), or Absent (DST gap); T is a FixedOffset or a DateTime[Tz]
Utc	The UTC zone: always offset zero
FixedOffset	A constant UTC offset, ±23:59:59
FixedZone	A named zone with one constant offset; zone_name/%Z report the name
DateTime[Tz]	A NaiveDateTime paired with a time zone Tz
Location	A time zone backed by parsed IANA tzdata, resolving historical transitions and DST
TransitionBounds	The validity window (start/end) of a Location's segment covering a given instant

#Quick start

Attaching a time zone to a UTC instant and reading it back in local time:

///|
test {
  let naive = @core.NaiveDateTime::new(
    @core.NaiveDate::from_ymd(2024, 6, 15).unwrap(),
    @core.NaiveTime::from_hms(12, 0, 0).unwrap(),
  )
  let dt = @tz.DateTime::from_utc(
    naive,
    @tz.FixedOffset::east(9 * 3600).unwrap(),
  )
  assert_eq(dt.offset(), @tz.FixedOffset::east(9 * 3600).unwrap())
  assert_eq(
    dt.naive_local(),
    @core.NaiveDateTime::new(
      @core.NaiveDate::from_ymd(2024, 6, 15).unwrap(),
      @core.NaiveTime::from_hms(21, 0, 0).unwrap(),
    ),
  )
}

DateTime::round/truncate operate on the underlying UTC instant, not the offset-shifted local presentation — truncating to an hour boundary in UTC can still leave a non-zero local minute under a non-whole-hour FixedOffset:

///|
test {
  let naive = @core.NaiveDateTime::new(
    @core.NaiveDate::from_ymd(2024, 6, 15).unwrap(),
    @core.NaiveTime::from_hms(12, 15, 0).unwrap(),
  )
  let dt = @tz.DateTime::from_utc(
    naive,
    @tz.FixedOffset::east(9 * 3600 + 1800).unwrap(),
  )
  let truncated = dt.truncate(@core.TimeDelta::hours(1L).unwrap()).unwrap()
  assert_eq(
    truncated.naive_utc(),
    @core.NaiveDateTime::new(
      @core.NaiveDate::from_ymd(2024, 6, 15).unwrap(),
      @core.NaiveTime::from_hms(12, 0, 0).unwrap(),
    ),
  )
  assert_eq(
    truncated.naive_local(),
    @core.NaiveDateTime::new(
      @core.NaiveDate::from_ymd(2024, 6, 15).unwrap(),
      @core.NaiveTime::from_hms(21, 30, 0).unwrap(),
    ),
  )
}

Looking up a real IANA zone by name and resolving its offset at a known DST transition:

///|
test {
  let ny = @tz.Location::load("America/New_York").unwrap()
  let dst_start_2024 = @core.NaiveDateTime::from_timestamp(1_710_054_000L, 0).unwrap()
  assert_eq(
    ny.offset_from_utc(dst_start_2024),
    @tz.FixedOffset::east(-14400).unwrap(),
  )
  assert_eq(ny.zone_name(dst_start_2024), "EDT")
}

#Embedded tz database

Location::load reads a snapshot of the IANA tz database compiled into the package: release 2026c, 598 zones, taken from the compiled rearguard data of a macOS system (/var/db/timezone/zoneinfo). Zone rules change after a release, so a zone that changed later (a government altering its DST rule) keeps its old rules until the snapshot is regenerated. The release is also named in the header of tzdata_generated.mbt.

To regenerate from another compiled zoneinfo tree, such as one built with zic from a specific release, run just gen-tzdata <zoneinfo dir>; the release is read from the tree's +VERSION file.

#API reference

#TimeZone trait

Implemented by Utc, FixedOffset, FixedZone, Location, and PosixTz. The trait is readonly: other packages can use Tz : TimeZone as a bound but cannot implement it, so these are the only zones. DateTime[Tz]::offset/naive_local require Tz : TimeZone.

Zone names have two roles with two spellings: name() (on Location and FixedZone) is the identifier that distinguishes the zone, such as the IANA id "Asia/Tokyo", while zone_name (on every TimeZone, and on DateTime) is the name in effect at an instant, such as the abbreviation "JST" or "EDT", "UTC", or a bare FixedOffset's offset text. DateTime::timezone() returns the zone value itself, as with_timezone takes one.

Every NaiveDateTime argument is a UTC reading except the one taken by offset_from_local, which is a local (wall-clock) reading. The type does not tell them apart, so pass DateTime::naive_utc() for the former and DateTime::naive_local() for the latter. To build a DateTime from either reading, use DateTime::from_utc/DateTime::from_local.

Method	Signature	Description
offset_from_utc(NaiveDateTime)	-> FixedOffset	The offset in effect at a given UTC instant; never ambiguous
offset_from_local(NaiveDateTime)	-> MappedLocalTime[FixedOffset]	The offset(s) for a given local (wall-clock) instant, handling DST ambiguity/gaps
zone_name(NaiveDateTime)	-> String	The zone abbreviation/name in effect at a given UTC instant
is_dst(NaiveDateTime)	-> Bool	Whether daylight saving time is in effect at a given UTC instant; false by default, overridden by Location, PosixTz and Local
transition_bounds(NaiveDateTime)	-> TransitionBounds	The validity window of the offset in effect at a given UTC instant; unbounded on both sides by default, overridden by Location (its own transition_bounds), PosixTz and Local
offset_from_abbreviation(String, NaiveDateTime)	-> FixedOffset?	The offset a zone abbreviation (e.g. "EST") denotes in this zone, resolved at a given UTC instant; None by default (Utc, FixedOffset), overridden by Location (see its own method), PosixTz (its standard and DST names) and Local

#MappedLocalTime[T]

///|
pub enum MappedLocalTime[T] {
  Single(T)
  Ambiguous(T, T) // a DST fold: two valid results, (earliest, latest)
  Absent // a DST gap: no valid result
}

Absent also covers components that do not form a valid reading (for example month 13) when the result is built from components (DateTime::from_ymd_hms, DateTime::with_*); to tell an invalid component from a gap, validate first with NaiveDate::from_ymd/NaiveTime::from_hms.

Utc and FixedOffset only ever produce Single (neither has daylight saving). A Location can produce all three around a real DST transition: Ambiguous when a local clock reading occurs twice (the fold at the end of DST), and Absent when a local clock reading never occurs (the gap at the start of DST). T is usually a FixedOffset (TimeZone::offset_from_local) or a DateTime[Tz] (DateTime::from_local/from_ymd_hms).

Method	Signature	Description
single()	-> T?	The value, only when unambiguous; None for Ambiguous/Absent
unwrap()	-> T	The value when unambiguous; aborts, naming the reason, on Ambiguous or Absent
earliest()	-> T?	The earliest possible value (the sole value, or the first of an Ambiguous fold); None for Absent
latest()	-> T?	The latest possible value (the sole value, or the second of an Ambiguous fold); None for Absent
map((T) -> U)	-> MappedLocalTime[U]	Transform every value carried by self, preserving its shape

MappedLocalTime[T] also implements Eq (when T : Eq).

There is deliberately no and_then. A step that itself returns a MappedLocalTime, applied to both sides of an Ambiguous, could yield up to four values with no obvious one to keep. Reduce to one value first with single(), earliest() or latest(), then continue; that choice stays with the caller:

///|
test {
  let dt = @tz.DateTime::from_ymd_hms(2024, 3, 5, 9, 0, 0, @tz.Utc::new()).unwrap()
  let moved = dt.with_hour(10).single().bind(d => d.with_minute(30).single())
  assert_eq(moved.map(d => d.hour() * 100 + d.minute()), Some(1030))
}

#Utc

The UTC zone, always offset zero.

Method	Signature	Description
Utc::new()	-> Self	Construct the (zero-sized) UTC zone value
Utc::now()	-> DateTime[Utc]	The current UTC instant, read from the host's wall clock. Unlike every other function in this package, not a pure function of its arguments. Total: a host clock outside NaiveDate's range (about 5.8 million years either side of 1970) is a broken environment, and aborts.
offset_from_utc(NaiveDateTime)	-> FixedOffset	Always FixedOffset::east(0)
offset_from_local(NaiveDateTime)	-> MappedLocalTime[FixedOffset]	Always Single(FixedOffset::east(0))
zone_name(NaiveDateTime)	-> String	Always "UTC"
to_string() (Show)	-> String	"UTC", the same text as zone_name

Utc also implements Eq and TimeZone.

#FixedOffset

A constant UTC offset, in seconds, within ±23:59:59.

Naming rule: a name with offset (DateTime::offset, offset_from_utc, offset_from_local) yields a FixedOffset value, while utc_offset and local_minus_utc yield the same quantity as a plain Int number of seconds.

Method	Signature	Description
FixedOffset::east(Int)	-> Self?	An offset east of UTC by the given seconds; None outside ±23:59:59
FixedOffset::west(Int)	-> Self?	An offset west of UTC by the given seconds (negated internally); same range
local_minus_utc()	-> Int	The offset in seconds (negative for a western offset)
utc_minus_local()	-> Int	The sign-reversed offset in seconds (positive for a western offset)
offset_from_utc(NaiveDateTime)	-> FixedOffset	Returns self, unchanged, regardless of the given instant
offset_from_local(NaiveDateTime)	-> MappedLocalTime[FixedOffset]	Always Single(self)
zone_name(NaiveDateTime)	-> String	Colon-separated sign, hour, and minute, e.g. "+09:00"; extended with a seconds component for a non-whole-minute offset, e.g. "-04:56:02"
to_string() (Show)	-> String	The same colon-separated text as zone_name, e.g. "+09:00"

FixedOffset also implements Eq and TimeZone.

#FixedZone

A named zone with a constant offset and no daylight saving. Unlike a bare FixedOffset, whose zone_name is its own offset text ("+09:00"), a FixedZone's zone_name (and so %Z) is the name it was given. Two zones are equal only when both the name and the offset match, so FixedOffset's own equality and rendering are unaffected.

Method	Signature	Description
FixedZone::new(String, FixedOffset)	-> Self	A zone with the given name and constant offset
name()	-> String	The zone's name
offset()	-> FixedOffset	The zone's constant offset
offset_from_utc(NaiveDateTime) / offset_from_local(NaiveDateTime)	-> FixedOffset / -> MappedLocalTime[FixedOffset]	The constant offset; the local form is always Single
zone_name(NaiveDateTime)	-> String	The zone's name
offset_from_abbreviation(String, NaiveDateTime)	-> FixedOffset?	The offset when the abbreviation equals the zone's own name, else None, so parse_date_time_in can read a %Z name back

#DateTime[Tz]

A NaiveDateTime paired with a time zone Tz. The UTC instant is stored directly; the local (wall-clock) representation is derived on demand.

Eq, Hash and Compare all identify the UTC instant and ignore the zone value, so the same instant in different zones is equal, hashes alike and compares as 0; compare the zones themselves (or naive_local()) when the wall-clock reading matters. Utc, FixedOffset, Location and TransitionBounds implement Hash too, so all of them can key a Map.

Method	Signature	Description
DateTime::from_utc(NaiveDateTime, Tz)	-> Self[Tz]	Wrap a UTC naive datetime with the given time zone
DateTime::unix_epoch()	-> Self[Utc]	The Unix epoch instant, 1970-01-01T00:00:00Z; also the Default for DateTime[Utc] (no other zone has a natural default)
DateTime::from_local(NaiveDateTime, Tz) (Tz : TimeZone)	-> MappedLocalTime[Self[Tz]]	Build from a local (wall-clock) naive datetime, resolving DST ambiguity via tz.offset_from_local
DateTime::from_local_lenient(NaiveDateTime, Tz) (Tz : TimeZone)	-> Self[Tz]	Like from_local but always succeeds: an unambiguous reading resolves exactly; a fold takes its first occurrence (as earliest()); a gap is read with the offset in effect just before the transition, landing after the gap by its length (02:30 in a 02:00-03:00 gap becomes 03:30)

The lenient rule is the library's only policy for a local reading that is repeated or skipped, and the calendar steps use it too. Other choices come from from_local (.latest() for the later occurrence of a fold, .single() to reject both cases). To build one from components, use NaiveDateTime::from_ymd_hms(..) and then DateTime::from_local_lenient. | DateTime::from_ymd_hms(Int, Int, Int, Int, Int, Int, Tz) (Tz : TimeZone) | -> MappedLocalTime[Self[Tz]] | Build from local calendar/time-of-day components; Absent for an invalid date/time-of-day, in addition to the usual DST-gap case | | DateTime::from_timestamp(Int64, Int, Tz) | -> Self[Tz]? | Build from a Unix timestamp (seconds + nanoseconds) through the given time zone; always unambiguous, None only on an out-of-range input | | DateTime::from_timestamp_millis(Int64, Tz) / from_timestamp_micros / from_timestamp_nanos | -> Self[Tz]? | Build from a Unix timestamp in that unit through the given time zone; None if the instant is outside NaiveDate's range (reachable only for milliseconds at Int64 extremes) | | naive_utc() | -> NaiveDateTime | The underlying naive datetime, in UTC | | timezone() | -> Tz | The time zone value this datetime is expressed in | | offset() (Tz : TimeZone) | -> FixedOffset | The UTC offset in effect at this instant | | naive_local() (Tz : TimeZone) | -> NaiveDateTime | The UTC datetime shifted by offset() | | with_timezone(Tz2) | -> Self[Tz2] | Re-express this datetime in Tz2, keeping the same UTC instant | | to_utc() | -> Self[Utc] | Re-express the same instant in UTC | | fixed_offset() | -> Self[FixedOffset] | Re-express the same instant with the offset in effect at that instant frozen (requires Tz : TimeZone); it no longer follows later DST changes | | add_signed(TimeDelta) | -> Self[Tz] | Advance by a signed duration, keeping the same time zone | | sub_signed(TimeDelta) | -> Self[Tz] | Move back by a signed duration | | add_offset(FixedOffset) / sub_offset(FixedOffset) | -> Self[Tz] | Move the instant forward/backward by an offset's seconds (the opposite way for a west offset), keeping the same time zone; abort if the result leaves NaiveDate's range | | checked_add_offset(FixedOffset) / checked_sub_offset(FixedOffset) | -> Self[Tz]? | The same, but None instead of aborting when out of range | | add_months(Int) (Tz : TimeZone) | -> Self[Tz] | Advance the local date by months, keeping the local time of day and the time zone; see NaiveDate::add_months (in core) for the day-of-month clamping rule. A result in a DST fold takes the earlier occurrence and one in a gap moves forward by the gap's length, as from_local_lenient does | | sub_months(Int) | -> Self[Tz] | Move the date back by months | | add_years(Int) (Tz : TimeZone) | -> Self[Tz] | Advance the local date by years, keeping the local time of day and the time zone; see NaiveDate::add_years (in core) for the clamping rule | | sub_years(Int) | -> Self[Tz] | Move the date back by years | | add_days(Int) (Tz : TimeZone) | -> Self[Tz] | Advance the local date by days, keeping the local time of day and the time zone, so a day across a DST change lasts 23 or 25 hours; fold and gap as for add_months | | sub_days(Int) | -> Self[Tz] | Move the date back by days | | add_seconds(Int64) / sub_seconds(Int64) | -> Self[Tz] | Move the instant by a whole number of seconds (backward if negative), keeping the same time zone; abort if the result leaves NaiveDate's range | | checked_add_seconds(Int64) / checked_sub_seconds(Int64) | -> Self[Tz]? | The same, but None instead of aborting when out of range | | checked_add_signed(TimeDelta) / checked_sub_signed(TimeDelta) | -> Self[Tz]? | Like add_signed/sub_signed, but None instead of aborting when the result leaves NaiveDate's range | | checked_add_months(Int) / checked_sub_months(Int) / checked_add_years(Int) / checked_sub_years(Int) / checked_add_days(Int) / checked_sub_days(Int) | -> Self[Tz]? | Like the aborting forms above, but None instead of aborting when the date leaves NaiveDate's range | | date() / time() | -> NaiveDate / -> NaiveTime | The local calendar date / time of day | | year() / month() / day() / ordinal() / weekday() / iso_week() | -> Int / -> Month / -> Int / -> Int / -> Weekday / -> IsoWeek | Local calendar components, derived from naive_local() (requires Tz : TimeZone) | | ymd() / hms() | -> (Int, Month, Int) / (Int, Int, Int) | The local date and time components together | | leap_year() | -> Bool | Whether the local calendar year is a leap year | | hour() / minute() / second() / nanosecond() | -> Int | Local time-of-day components; a leap second reports second() == 59 with nanosecond() >= 1_000_000_000 | | month0() / day0() / ordinal0() / quarter() / num_days_in_month() / num_days_from_ce() | -> Int | Further local calendar components (0-based month/day/ordinal, quarter 1..=4, month length, days since the start of the Common Era) | | year_ce() / hour12() | -> YearCe / -> ClockHour12 | Local year as a Common Era flag plus a positive year; local hour as a PM flag plus an hour in 1..=12 (@core.YearCe, @core.ClockHour12) | | num_seconds_from_midnight() | -> Int | Seconds since local midnight | | with_year(Int) / with_month(Int) / with_day(Int) / with_ordinal(Int) / with_hour(Int) / with_minute(Int) / with_second(Int) / with_nanosecond(Int) | -> MappedLocalTime[Self[Tz]] | Replace one local component and re-resolve the wall-clock reading through the zone: Absent for an invalid value or a DST gap, Ambiguous inside a DST fold | | with_month0(Int) / with_day0(Int) / with_ordinal0(Int) | -> MappedLocalTime[Self[Tz]] | 0-based counterparts of with_month/with_day/with_ordinal, resolved the same way | | with_date(NaiveDate) | -> MappedLocalTime[Self[Tz]] | Replace the local date, keeping the local time of day; Absent inside a DST gap, Ambiguous inside a fold | | with_time(NaiveTime) | -> MappedLocalTime[Self[Tz]] | Replace the local time of day, keeping the local date, resolved the same way as the with_* setters | | timestamp() / timestamp_millis() | -> Int64 | Non-leap seconds / milliseconds since the Unix epoch, flooring toward negative infinity; independent of the zone | | timestamp_micros() / timestamp_nanos() | -> Int64? | Microseconds / nanoseconds since the Unix epoch; None if the instant overflows Int64 | | timestamp_subsec_nanos() / timestamp_subsec_millis() / timestamp_subsec_micros() | -> Int | The sub-second component of the instant in that unit | | signed_duration_since(Self[Tz2]) | -> TimeDelta | The signed duration from other to self, independent of either's time zone | | zone_name() | -> String | The zone's name at this instant: an IANA abbreviation ("EDT"), a FixedZone's name, "UTC", or a bare FixedOffset's offset text; the same text as %Z (requires Tz : TimeZone) | | is_dst() | -> Bool | Whether daylight saving time is in effect at this instant (requires Tz : TimeZone) | | zone_bounds() | -> TransitionBounds | The validity window of the offset in effect at this instant, either side None when unbounded (requires Tz : TimeZone); see TransitionBounds | | to_string() (Show) | -> String | The local date-time and the zone's name at that instant joined by a space, e.g. "2024-01-02 13:45:06.500 +09:00", "... UTC", "... EDT" (requires Tz : TimeZone) | | years_since(Self[Tz2]) | -> Int? | Full calendar years elapsed from base (which may be in another time zone), reading both as local dates in the receiver's time zone and ignoring the time of day; None if base is later | | compare(Self[Tz]) / < / > / <= / >= | -> Int / -> Bool | Order by UTC instant, ignoring the zone value; consistent with == | | compare_instant(Self[Tz2]) | -> Int | Order by UTC instant against a datetime in a different time zone type | | round(TimeDelta) | -> Result[Self[Tz], @core.RoundingError] | Round the underlying UTC instant to the nearest multiple of a granularity since the Unix epoch, keeping the same time zone; see TimeDelta::round (in core) for which granularities are supported | | round_up(TimeDelta) | -> Result[Self[Tz], @core.RoundingError] | Round the underlying UTC instant up (toward positive infinity) to the next multiple of a granularity since the Unix epoch, unchanged if already a multiple, keeping the same time zone; see NaiveDateTime::round_up for the failure reasons | | truncate(TimeDelta) | -> Result[Self[Tz], @core.RoundingError] | Truncate the underlying UTC instant toward the Unix epoch; see Quick start above for how this differs from truncating the local presentation | | round_subsecs(Int) / truncate_subsecs(Int) | -> Result[Self[Tz], @core.RoundingError] / -> Self[Tz] | Round or truncate the underlying UTC instant to a number of fractional-second digits (0..=9; other values abort) |

Rounding acts on the UTC instant, not the local reading, so in a zone such as +05:30 a one-hour truncation leaves a local time that is not on the hour. To round to a local boundary, round naive_local() and resolve the result with DateTime::from_local. Calendar steps (add_days, add_months, add_years) act on the local reading, while add_signed, add_seconds and the rounding methods act on the instant.

DateTime[Tz] also implements Eq (when Tz : Eq).

#Location

A time zone backed by parsed IANA tzdata (TZif binary format, plus a POSIX TZ string for extrapolating past the last recorded transition).

Method	Signature	Description
Location::load(String)	-> Self?	Look up an embedded IANA zone by name (e.g. "Asia/Tokyo"), following aliases; None if the name is unknown, including the empty string (which is not an alias for UTC); "UTC" resolves like any other zone
Location::utc()	-> Self	The UTC zone as a Location (equal to Location::load("UTC"), name() is Some("UTC")), for APIs taking a Location rather than the separate Utc type
Location::from_tzif_bytes(Bytes)	-> Self?	Parse a zone directly from raw TZif bytes; None if malformed
Location::from_tzif_bytes_named(String, Bytes)	-> Self?	Like from_tzif_bytes, but name() reports the given name; any text is accepted as given, without validation
name()	-> String?	The IANA identifier this Location was loaded with (the name as given to Location::load, not canonicalized through an alias); None for one built via from_tzif_bytes
to_string() (Show)	-> String	name(), or an empty string for a Location without one
offset_from_abbreviation(String, NaiveDateTime)	-> FixedOffset?	The offset an abbreviation (e.g. "EST") denotes: that of the type in effect at the given UTC instant if it matches, else of the first type in the zone's table with that abbreviation; None if none has it
type_at(NaiveDateTime)	-> LocalTimeType	The offset, DST flag, and abbreviation in effect at a given UTC instant
offset_from_utc(NaiveDateTime)	-> FixedOffset	The offset in effect at a given UTC instant
offset_from_local(NaiveDateTime)	-> MappedLocalTime[FixedOffset]	The offset(s) for a given local instant, resolving DST folds and gaps
zone_name(NaiveDateTime)	-> String	The abbreviation in effect at a given instant, e.g. "EDT"
transition_bounds(NaiveDateTime)	-> TransitionBounds	The validity window of the segment covering a given instant; see TransitionBounds

Location also implements TimeZone. == is structural: two Locations are equal when their parsed TZif data, POSIX rule and name() all match, so an alias (e.g. "Japan") is unequal to its canonical zone ("Asia/Tokyo") even though both resolve identically.

#TransitionBounds

The validity window of a Location's segment covering a given instant, as returned by Location::transition_bounds: the local time type type_at reports is in effect from start() (inclusive) until end() (exclusive). None on either side means unbounded in that direction.

Method	Signature	Description
start()	-> NaiveDateTime?	The instant this segment began; None if unbounded (before the zone's first recorded transition, or for a zone with no transitions at all)
end()	-> NaiveDateTime?	The instant the next segment begins; None if unbounded (past the zone's last recorded transition, when no POSIX rule extrapolates further)

Past the last recorded transition, the POSIX rule's own bounds are computed exactly (not approximated near a year boundary).

TransitionBounds also implements Eq.

#LocalTimeType

The offset, DST flag, and abbreviation for one segment of a Location's timeline, as returned by Location::type_at.

Method	Signature	Description
LocalTimeType::new(Int, Bool, String)	-> Self?	Construct from UTC offset (seconds), DST flag, and abbreviation; None if the offset is outside ±86399 (±23:59:59), so every LocalTimeType has an offset a FixedOffset can hold
utc_offset()	-> Int	UTC offset in seconds
is_dst()	-> Bool	Whether daylight saving is in effect
abbreviation()	-> String	The zone abbreviation, e.g. "EST"/"EDT"

LocalTimeType also implements Eq.

#Local (native only)

The OS-configured local time zone, resolved from $TZ or, when unset, /etc/localtime. Only compiled for the native backend: resolving it requires file I/O that js/wasm/wasm-gc have no host-provided access to. The generated .mbti of the default target (wasm) therefore omits Local; its native interface (Local::new, Local::now and the TimeZone methods) is the one to review for a release. The one place the host environment is read is Local::new; the rule it applies to $TZ and /etc/localtime is internal and not public API.

Method	Signature	Description
Local::new()	-> Self?	Resolves the host's configured time zone; None if it could not be determined. Reads live OS state — not a pure function of its arguments.
Local::now()	-> DateTime[Local]?	The current instant in the host's local zone, the counterpart of Utc::now(); None when Local::new() is None, that is, when the zone cannot be determined; the clock itself is read as in Utc::now()
offset_from_utc(NaiveDateTime)	-> FixedOffset	Delegates to the resolved zone
offset_from_local(NaiveDateTime)	-> MappedLocalTime[FixedOffset]	Delegates to the resolved zone
zone_name(NaiveDateTime)	-> String	Delegates to the resolved zone

Local also implements TimeZone. $TZ may be an IANA zone name, an empty string (UTC), or a bare POSIX TZ rule such as "JST-9" or "FOO5BAR4,M3.2.0,M11.1.0"; an IANA name takes precedence when a string is both (e.g. "EST5EDT"). The leading-colon form (":Asia/Tokyo") is not handled.

#Advanced: low-level TZif and POSIX TZ parsing

Failure is reported as None throughout this package (parse_tzif, parse_posix_tz, Location::from_tzif_bytes, Location::from_tzif_bytes_named, Location::load), unlike format, whose parsers raise a FormatError naming the reason. These read machine data (TZif bytes, POSIX TZ strings, zone names), where a caller's response to any failure is the same, so no reason is carried; each function's documentation lists the conditions under which it returns None (a malformed structure, an offset beyond ±23:59:59, or an unknown zone name).

These back Location and are not usually needed directly; use Location::load/from_tzif_bytes unless building a custom zone-data pipeline.

Symbol	Signature	Description
parse_tzif(Bytes)	-> TzifData?	Parse raw TZif bytes (header, transition table, local-time-type table, leap seconds, POSIX TZ footer)
parse_posix_tz(String)	-> PosixTz?	Parse a POSIX TZ rule string (all three date-rule forms: Jn, n, Mm.w.d)

Type	Key methods	Description
TzifData	transitions(), transition_types(), local_time_types(), leap_seconds(), posix_tz()	The parsed contents of a TZif file; the array accessors return copies, and posix_tz() is the already-parsed PosixTz? footer (None when empty or absent; a malformed one makes parse_tzif return None)
PosixTz	type_at(NaiveDateTime) (a UTC reading, like Location::type_at), offset_from_local(NaiveDateTime)	An evaluated POSIX TZ rule, for extrapolating past a TZif file's last recorded transition; also a TimeZone in its own right (a constant offset when the rule has no DST part, e.g. JST-9)

#tz package

#Key types

#Quick start

#Embedded tz database

#API reference

#TimeZone trait

#MappedLocalTime[T]

#Utc

#FixedOffset

#FixedZone

#DateTime[Tz]

#Location

#TransitionBounds

#LocalTimeType

#Local (native only)

#Advanced: low-level TZif and POSIX TZ parsing

#TimeZone

#DateTime

#DateTime::add_days

#DateTime::add_months

#DateTime::add_offset

#DateTime::add_seconds

#DateTime::add_signed

#DateTime::add_years

#DateTime::checked_add_days

#DateTime::checked_add_months

#DateTime::checked_add_offset

#DateTime::checked_add_seconds

#DateTime::checked_add_signed

#DateTime::checked_add_years

#DateTime::checked_sub_days

#DateTime::checked_sub_months

#DateTime::checked_sub_offset

#DateTime::checked_sub_seconds

#DateTime::checked_sub_signed

#DateTime::checked_sub_years

#DateTime::compare

#DateTime::compare_instant

#DateTime::date

#DateTime::day

#DateTime::day0

#DateTime::default

#DateTime::equal

#DateTime::fixed_offset

#DateTime::from_local

#DateTime::from_local_lenient

#DateTime::from_timestamp

#DateTime::from_timestamp_micros

#DateTime::from_timestamp_millis

#DateTime::from_timestamp_nanos

#DateTime::from_utc

#DateTime::from_ymd_hms

#DateTime::hash

#DateTime::hash_combine

#DateTime::hms

#DateTime::hour

#DateTime::hour12

#DateTime::is_dst

#DateTime::iso_week

#DateTime::leap_year

#DateTime::minute

#DateTime::month

#DateTime::month0

#DateTime::naive_local

#DateTime::naive_utc

#DateTime::nanosecond

#DateTime::not_equal

#DateTime::num_days_from_ce

#DateTime::num_days_in_month

#DateTime::num_seconds_from_midnight

#DateTime::offset

#DateTime::op_ge

#DateTime::op_gt

#DateTime::op_le

#DateTime::op_lt

#DateTime::ordinal

#DateTime::ordinal0

#DateTime::output

#DateTime::quarter

#DateTime::round

TimeZone

DateTime

DateTime::add_days

DateTime::add_months

DateTime::add_offset

DateTime::add_seconds

DateTime::add_signed

DateTime::add_years

DateTime::checked_add_days

DateTime::checked_add_months

DateTime::checked_add_offset

DateTime::checked_add_seconds

DateTime::checked_add_signed

DateTime::checked_add_years

DateTime::checked_sub_days

DateTime::checked_sub_months

DateTime::checked_sub_offset

DateTime::checked_sub_seconds

DateTime::checked_sub_signed

DateTime::checked_sub_years

DateTime::compare

DateTime::compare_instant

DateTime::date

DateTime::day

DateTime::day0

DateTime::default

DateTime::equal

DateTime::fixed_offset

DateTime::from_local

DateTime::from_local_lenient

DateTime::from_timestamp

DateTime::from_timestamp_micros

DateTime::from_timestamp_millis

DateTime::from_timestamp_nanos

DateTime::from_utc

DateTime::from_ymd_hms

DateTime::hash

DateTime::hash_combine

DateTime::hms

DateTime::hour

DateTime::hour12

DateTime::is_dst

DateTime::iso_week

DateTime::leap_year

DateTime::minute

DateTime::month

DateTime::month0

DateTime::naive_local

DateTime::naive_utc

DateTime::nanosecond

DateTime::not_equal

DateTime::num_days_from_ce

DateTime::num_days_in_month

DateTime::num_seconds_from_midnight

DateTime::offset

DateTime::op_ge

DateTime::op_gt

DateTime::op_le

DateTime::op_lt

DateTime::ordinal

DateTime::ordinal0

DateTime::output

DateTime::quarter

DateTime::round

DateTime::round_subsecs

DateTime::round_up

DateTime::second

DateTime::signed_duration_since

DateTime::sub_days

DateTime::sub_months

DateTime::sub_offset

DateTime::sub_seconds

DateTime::sub_signed

DateTime::sub_years

DateTime::time

DateTime::timestamp

DateTime::timestamp_micros

DateTime::timestamp_millis

DateTime::timestamp_nanos

DateTime::timestamp_subsec_micros