Module datetime 

Pure-arithmetic datetime manipulation (epoch-based, no system clock). DateTime support for CJC.

Design decisions:

• Epoch millis (i64), UTC only — deterministic, no timezone ambiguity
• datetime_now() is NONDET (uses system clock)
• All other operations are pure arithmetic on epoch millis
• Leap year handling for year/month/day extraction

Functions

datetime_add_millis

Add milliseconds to a datetime.

datetime_day

Extract the day of month (1-31) from epoch millis.

datetime_diff

Difference between two datetimes in milliseconds.

datetime_format

Format a datetime as ISO 8601 UTC string: YYYY-MM-DDTHH:MM:SSZ

datetime_from_epoch

Create a datetime from epoch milliseconds (identity, but validates type).

datetime_from_parts

Create a datetime from year, month, day, hour, minute, second components. All components are 1-based for month/day.

datetime_hour

Extract the hour (0-23) from epoch millis.

datetime_minute

Extract the minute (0-59) from epoch millis.

datetime_month

Extract the month (1-12) from epoch millis.

datetime_now

Returns current UTC time as epoch milliseconds. This is NONDET — the only nondeterministic datetime operation.

datetime_second

Extract the second (0-59) from epoch millis.

datetime_year

Extract the year from epoch millis.

days_in_month

Return the number of days in the given month (1-12) of the given year.