fdate is a command line utility written in rust that outputs the date according to the french republican calendar and the time in decimal time. It has formatting options closely resembling the date command.

It is available at github.com/nyabla/fdate

My journey in implementing this was a rough one, with many false starts and a lot of research. I started with reading up on the calendar and deciding that I wanted to implement astronomical calculations by hand, with the help of Meeus's Astronomical Algorithms[1], which I very quickly came to regret.

With this insight I employed the help of the astro crate but I just couldn't get it to spit out the correct numbers so I got back on the research grind. I managed to stumble across a general algorithm for converting between julian dates to arbitrary calendars and back[2]. This led me on a search to find the coefficients for the french republican calendar which, surprisingly, existed[3] so I was saved and could finish off my silly little project.

I did end up using good ol' Meeus for calculating the julian date and did some simple maths to convert the time into decimal time. Also I yoinked Don Knuth's roman numeral implementation from tex.web lol.


  1. ^ the 1991 edition of Astronomical Algorithms is available on the internet archive
  2. ^ Generalized Equations for Julian Day Numbers and Calendar Dates (1985) by D. A. Hatcher is available online from the astrophysics data system
  3. ^ Additif to 'Generalized Equations for Julian Day Numbers and Calendar Dates' (1986) by J. P. Parisot is also available online from the astrophysics data system