$ date –date=”-1000 weeks ago”
Fri May 21 18:03:28 PDT 2032

$ date –date=”+2 weeks ago”
Fri Mar 8 17:01:46 PST 2013

$ date –date=”-1000 days ago”
Thu Dec 17 17:07:07 PST 2015

can use seconds, minutes, hours, days, weeks, months. string can be “next Saturday”, “1 month ago”, “calendar date, time of day, time zone, day of week, relative time, relative date, and numbers. “yesterday”. “tomorrow”, can use negatives “-1 day ago”, “next day”,

Going into future: “tomorrow”, “1 day”, “1 month”, “fortnight” (14 days), “3rd Friday”
back in time: “1 day ago”, “69 weeks ago”, “34 years ago”

$ date +”%d/%m/%y” –date=”-84 weeks ago”
31/10/14

$ date –date=”next Thursday”
Thu Mar 28 00:00:00 PDT 2013