gregorian 
An implementation of the proleptic Gregorian calendar, compatible with ISO 8601. Amongst others, that means that the calendar has a year zero preceeding the year 1.
This create does not deal with times or time zones.
The Date type represents a date (year, month and day),
the Year type represents a calendar year,
the Month type represents a calendar month,
and the YearMonth type represents a month of a specific year.
Where possible, things are implemented as const fn.
Currently, this excludes trait implementations and functions that rely on traits.
Example
use gregorian::{Date, Month::*, Year, YearMonth};
assert!(Year::new(2020).has_leap_day(), true);
assert!(YearMonth::new(1900, February).total_days() == 28);
assert!(YearMonth::new(2000, February).total_days() == 29);
assert!(Year::new(2020).with_month(March).first_day() == Date::new(2020, March, 1).unwrap());
assert!(Year::new(2020).with_month(March).last_day() == Date::new(2020, March, 31).unwrap());
assert!(Year::new(2020).first_day() == Date::new(2020, 1, 1).unwrap());
assert!(Year::new(2020).last_day() == Date::new(2020, 12, 31).unwrap());
assert!(Date::new(2020, 2, 1).unwrap().day_of_year() == 32);Rounding invalid dates
When you use Date::add_years() or Date::add_months(), you can get invalid dates.
These are reported with an InvalidDayOfMonth error which has the
next_valid() and prev_valid() methods.
Those can be used to get the next or previous valid date instead.
Additionally, there is an extension trait for Result<Date, InvalidDayOfMonth> with the
or_next_valid() and or_prev_valid() methods.
This allows you to directly round the date on the Result object.
use gregorian::{Date, DateResultExt};
let date = Date::new(2020, 1, 31).unwrap();
assert!(date.add_months(1).or_next_valid() == Date::new(2020, 3, 1).unwrap());
assert!(date.add_months(1).or_prev_valid() == Date::new(2020, 2, 29).unwrap());