How do I do date math in a bash script on OS X Leopard?

For example, this OS X date command will add 14 days to the input string 20160826 (not the present time):

date -j -f %Y%m%d -v+14d 20160826 +%Y%m%d

$ date -v -1d

-d sets Daylight Savings time flag.

Try man date for more info.