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julian is a Rust library for converting between Julian day numbers and dates in the Gregorian calendar (either proleptic or with the Reformation occurring at a given date) and/or the proleptic Julian calendar. There are also features for querying details about years & months in a "reforming" Gregorian calendar and how they are affected by the calendar reformation date of your choice.

Examples

Before you construct a date, you must first choose a calendar in which to reckon dates. Calendar::GREGORIAN is the proleptic Gregorian calendar, which should be both simple and useful enough for most basic purposes.

To convert a Julian day number to a date in a calendar, use the Calendar::at_jdn() method, like so:

use julian::{Calendar, Month};

let cal = Calendar::GREGORIAN;
let date = cal.at_jdn(2460065);
assert_eq!(date.year(), 2023);
assert_eq!(date.month(), Month::April);
assert_eq!(date.day(), 30);

So JDN 2460065 is April 30, 2023, in the proleptic Gregorian calendar.

To convert a date to a Julian day number, use Calendar::at_ymd() to construct the date, and then call its julian_day_number() method:

use julian::{Calendar, Month};

let cal = Calendar::GREGORIAN;
let date = cal.at_ymd(2023, Month::April, 30).unwrap();
assert_eq!(date.julian_day_number(), 2460065);

See the documentation for more things you can do!

Command

There is also an accompanying binary package julian-cli that provides a CLI command named julian for converting between Julian day numbers and dates in Julian-style calendars. Feel free to install it if you're interested!