TOML datetime variants (RFC 3339 subset).
TOML distinguishes four kinds of datetime literals:
- OffsetDateTime — full date + time + UTC offset (e.g. 1979-05-27T07:32:00Z)
- LocalDateTime — date + time, no offset (e.g. 1979-05-27T07:32:00)
- LocalDate — date only (e.g. 1979-05-27)
- LocalTime — time only (e.g. 07:32:00)
Values are stored as their original string form. The parser preserves
the spelling supplied in the TOML source; it does not normalize or
validate the underlying calendar arithmetic.