UNIX for Advanced & Expert Users

At the high level, 50 days back from last Friday is 1) find the day of week. If it is Friday, add 7 to 50 and now it is in "days back" (or 1 to 6 as appropriate for the day of the week. Be careful about day of week zero or one based and week systems starting on different days!) Now calculate that date and format it.

If you want to write your own calendar code, the best way is to move your epoch form 1/1/1970 to 3/1/1968 or any othe day right after a leap year day. Now, every dats is a number of 4 year leaps plus 0-3 365 day years plus 0-11 months. You can subtract 365 day years and normal day count months ony. February is never subtracted, as it is at the top of every 4 year cycle, and its day count is just what remains. You only have to worry about the 100/400 year thing if you will calculate outside 1901-2099. Converting to Julian is tougher, especially with the short, rent riot month! Lunar calendars have their own rules, usually aligning to a solar-earth event like a solstice, and have to dispose of loose change somewhere. Those seconds they dump in the clock now and then as the earth slows down are only an issue to astronomers and others that deal in really precise time over long intervals. Leap second - Wikipedia, the free encyclopedia

One really messy aspect is determining DST in the past. I recall never finding a good way to deal with shift changes shifting when writing per-shift aggregate reports for nominally 24x7 wafer fab shifts. It did not occur to them to start the shifts the same EST time all year, so day shift might start at 7 or 8, for instance. Maybe that was against DST law? Even if you know where thr 9 and 7 hour shifts are, it is a mess when they are aggregated in weeks!

Usually, you find or write a tool or import a library. Google shell time date arithmetic and get: https://www.unix.com/answers-frequent...rithmetic.html