I was looking at Tomohiko Sakamoto's weekday calculator. It's a formula to calculate the day-of-week directly given year, month, day. That made me wonder what other neat date calculation shortcuts exist.
In particular, given an input date as (in_year, in_month, in_day) and a number of days N to add, what's a formula for returning the output (out_year, out_month, out_day)? Is there a well-known trick like the algorithm above?
One way would be to convert the input to a Julian day (a count of days since 4713 BC), add N to it, and then convert back. There are formulas for conversion in both directions. But the combined formula would be quite unwieldy. Is there a simplified version?
Perhaps there is even a formula to move forward or back by a certain number of weekdays.
This question isn't "how do I do date arithmetic in my favourite programming language?" I know how to call the date library to perform these operations. It's more curiosity and the hope of starting a collection of cool date algorithms.
