How do I get the bash date script to return a day of the week relative to a non-current time?

Question

Using bash date, I can get it to return a day of the week relative to the current time.

date --d='last Sunday' #Returns date of the Sunday before today

I can also get it to return a day relative to some other date

date --d='02/1/2012 -2 days' #Returns date two days before Feb. 1, 2012

but how can I get it to return the day of the week relative to some non-current time? What I want to do is:

date --d='Sunday before 02/1/2012' #Doesn't work! I want Sunday before Feb. 1

If possible, I would even like to be able to chain strings so that I can reference relative days from the new date:

# Should return 2 days before the Sunday before Feb. 1, 2012
date --d='Sunday before 02/1/2012 - 2 days'

Though this chaining is not as important. Is there some way for bash date to return a day based on the relative day of the week?

Answer 1

You can use a little day number arithmetic:

base="02/1/2012"
feb1_dayofweek=$( date -d $base +%w )
target_dayofweek=0   # sunday
date -d "$base - $(( (7 + feb1_dayofweek - target_dayofweek) % 7 )) days"

result:

Sun Jan 29 00:00:00 EST 2012