Date arithmetic in Unix shell scripts

Question

I need to do date arithmetic in Unix shell scripts that I use to control the execution of third party programs.

I\'m using a function to increment a day and another

Answer 1

date --date='1 days ago' '+%a'

It's not a very compatible solution. It will work only in Linux. At least, it didn't worked in Aix and Solaris.

It works in RHEL:

date --date='1 days ago' '+%Y%m%d'
20080807