Crate gregorian

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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.


use gregorian::{Date, Month::*, Year, YearMonth};

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());


pub use Month::*;


A calendar date consting of a year, month and day.
The string does not follow the proper date syntax.
The day is not valid for the year and month.
The day-of-year is not valid for the year.
The month number is not valid.
A calendar year.
A month of a specific year.


The string is not a valid date.
The date is not valid.
A month on the Gregorian calendar.


All months in order as array.


Extension for Result<Date> to round invalid dates.