If you need to work with dates in a shell script, this oneline could be useful:
echo $((`date +%s` - $OFFSET))|awk '{print strftime("%Y-%m-%d",$1)}'
What it does: it takes the current system time, converts it to epoch, rest $OFFSET (which should be in seconds) and then convert it again in the forma YYYY-MM-DD (but you can use every output supported by strftime, man strftime). Useful if you want to do some quick date calculation without having to fight with months, leap years and so on.
The eternal fight between admins and computers 
Nice trick — I didn’t know about awk’s strftime. But you can do some of the same things with GNU’s date too:
$ date -d ‘2 seconds ago’ +’%Y-%m-%d-%H-%M-%S’
2007-09-04-06-22-48
$ date -d ‘+4 days’ +’%Y-%m-%d-%H-%M-%S’
2007-09-08-06-23-14
I’m definitely going to remember strftime for when I don’t have GNU date, though.