Has a pretty good summary of using GNU date’s strtotime implementation.
strtotime.y is an interesting piece of code. Its often reproduced and imitated. The header says
Originally written by Steven M. Bellovin <email@example.com> while at the University of North Carolina at Chapel Hill. Later tweaked by a couple of people on Usenet. Completely overhauled by Rich $alz <firstname.lastname@example.org> and Jim Berets <email@example.com> in August, 1990;
This grammar has 13 shift/reduce conflicts.
This code is in the public domain and has no copyright.
It turns out GNU renames this to parse-datetime.y, and fixes some local DST issues, but you can see that original message still there.
Its a nice lex/yacc refresher when you have been away from those tools for a while, and a nice C refresher too. Using it is easier than understanding how it works.
One of the things I like is that you can combine expressions.
$ date -d ‘1 day ago’
Wed May 14 15:11:51 UTC 2014
Just leave out the english conjunction. So instead of 1 day ago and 2 hours ago, say 1 day ago 2 hours ago.
$ date -d ‘1 day ago 2 hours ago’
Wed May 14 13:12:00 UTC 2014
I should mention that these are correct, because the time right now is
Thu May 15 15:12:50 UTC 2014
One thing which is not really clear in the above tip page is that minus is just an alias for ago.
$ date -d ‘-1 day -2 hours’
Wed May 14 13:14:04 UTC 2014
Things one may wish to do is floor a result. e.g. making yesterday start at top of yesterday.
$ date -d ‘yesterday 00:00′
Wed May 14 00:00:00 UTC 2014
Finally, I was surprised to dig up ruby’s date_parse.c and find that it does not claim any heritage with the original strtotime.y.
update 3 hours later:
I was just doing some comparisons on systems which use strtotime and the question arose, what if I call it with empty string?
$ date -d ”
Thu May 15 00:00:00 UTC 2014
easy answer: its the midnight floor of todays date. The same as ‘now 00:00:00′, ‘now 00:00′, or just ‘now 0′.