Shell Scripting Date manipulation

Hi Experts,

i have date as inputdate=01/01/2013,how to get the previous date from this date and also first day's date of the month.

example:

inputdate=01/06/2013

previousdate=31/05/2013

firstdate=01/05/2013

how can i get solution to this. my unix is not supporting GNU Dates

regrads,