Add date and time handling with ISO 8601 support.

Motivation

Timestamp comparison, duration calculation, and date formatting are needed for any data processing, logging, scheduling, or API interaction. ISO 8601 is the universal interchange format.

Proposed functions

• now(-> @Int) effects(<IO>) — current Unix timestamp in milliseconds
• datetime_parse(@String -> Result<@DateTime, @String>) — parse ISO 8601 string
• datetime_format(@DateTime -> @String) — format as ISO 8601
• datetime_add(@DateTime, @Int -> @DateTime) — add milliseconds
• datetime_diff(@DateTime, @DateTime -> @Int) — difference in milliseconds

Implementation notes

DateTime is a host-provided type backed by milliseconds since Unix epoch. Timezone handling via IANA timezone database at the host level.

Dependencies

• None