Simple implementation of the Gregorian calendar for Rust.

Gregorian rules are used for all time, which is not historically accurate before 1583.

• UnixTimestamp represents an instant as a (possibly negative) integer number of seconds since the Unix Epoch, January 1st 1970 at midnight UTC. (There is no sub-second resolution.)
• NaiveDateTime represents a date in the Gregorian calendar with components year, month, day, hour, minute, and second. By itself, it can not be interpreted as an instant in time.
• DateTime is a NaiveDateTime together with a time zone.

By default the crate uses #![no_std] so that it can be used in freestanding environments. If the system_time Cargo feature is enabled, it uses std to implement conversions to and from std::time::SystemTime.