How can process a date that is in d/m/yyyy?

How can process a date that is in d/m/yyyy?

I have to process dates . If the format i specify is mm/dd/yyyy and the
date is like 7/5/2013, it is throwing a format exception. Does the date
has to be like 07/05/2013? If yes how can i change the date from 7/5/2013
to 07/05/2013 programatically ? and the date format is not specific. I can
have dates in mm-dd-yyyy or yyyy-mm-dd and the dates can come like
7-5-2013.