You might be responding tongue-in-cheek, but you have essentially described an underspecified variant of FDN Rata Die [1]. FDN R.D. does have a place in calend

You might be responding tongue-in-cheek, but you have essentially described an underspecified variant of FDN Rata Die [1]. FDN R.D. does have a place in calendrical calculations; namely, it serves as the narrow waist for converting between calendars. All the best, [1]: https://en.wikipedia.org/wiki/Rata_Die https://en.wikipedia.org/wiki/Rata_Die