If today was 2014-08-03 and you wanted the date three days in the past try–
reldate 3 days
The output would be
Supported time units are
Specifying a date to calucate from
reldate handles dates in the YYYY-MM-DD format (e.g. March 1, 2014 would be 2014-03-01). By default reldate uses today as the date to calculate relative time from. If you use the –from option you can it will calculate the relative date from that specific date.
reldate --from=2014-08-03 3 days
Command line arguments traditionally start with a dash which we also use to denote a nagative number. To tell the command line process that to not treat negative numbers as an “option” precede your time increment and time unit with a double dash.
reldate --from=2014-08-03 -- -3 days
You can calculate a date from a weekday name (e.g. Saturday, Monday, Tuesday) knowning a day (e.g. 2015-02-10 or the current date of the week) occurring in a week. A common case would be wanting to figure out the Monday date of a week containing 2015-02-10. The week is presumed to start on Sunday (i.e. 0) and finish with Saturday (e.g. 6).
reldate --from=2015-02-10 Monday
As that is the Monday of the week containing 2015-02-10. Weekday names case insensitive and can be the first three letters of the English names or full English names (e.g. Monday, monday, Mon, mon).