The [date] program in Linux is incredibly powerful, and can be used to modify dates very quickly. Here are some examples.
The current time in my present locale:
$ date Thu Nov 13 15:41:12 MST 2014
The current time in UTC: (Use UTC, not GMT. One is an international standard of keeping time based on atomic clocks while the other is a local old-fashioned timezone based on when the sun is highest in the sky … which isn’t exactly accurate enough for international business. Plus, GMT isn’t even used when daylight saving time is in effect. Anyway…)
$ date --utc Thu Nov 13 22:44:06 UTC 2014
The time one day ago in UTC:
$ date --utc -d "now -1 day" Wed Nov 12 22:44:51 UTC 2014
A specific date minus one day, formatted as we wish:
date -d "2014-10-01 -1 day" +%Y-%m-%d 2014-09-30
See the manual page for date for more options.