[SOLVED] how to find date of nth occurrence of a particular day of the week (e.g. Sunday) in a particular month in gnu date bash

how to find date of nth occurrence of a particular day of the week (e.g. Sunday) in a particular month in gnu date bash

I want to find the date for the third Sunday of the month for a given month using the gnu date command in bash.

I know that one can write this to find the next Sunday from now:

 date +%Y%m%d -d "next sunday"

(and could then add n weeks), but after reading the man page I don't see how to do this relative to a particular date, e.g. like

 date +%Y%m%d -d "20221001+next sunday"

or actually

 date +%Y%m%d -d "20221001+next sunday+2 weeks"

I've found this specific question answered for a lot of other languages on SO, or this similar question for gnu date in bash, but not this specific question using gnu date.

Solution

You could calculate it starting with the weekday %w of the first day in that month:

n=3        # The third
w=0        # Sunday (0=Sun .. 6=Sat)
ym=202411  # in November 24
d=$(((n-1)*7+(w-$(date +%w -d $ym'01')+7)%7+1))

echo $d    # gives 17