I would say such calculations are made much easier if time stamps are expressed in ISO format where all parts of the date are in order and numeric. 2012-04-12

I would say such calculations are made much easier if time stamps are expressed in ISO format where all parts of the date are in order and numeric. 2012-04-12 10:09 PST Since it is August 2012 now it is roughly 4 months ago (and one rarely needs more precision than this).