Just an attempt to get a more informative subject line--maybe somebody can improve it.
On Tuesday, February 06, 2018 08:49:39 AM Greg Wooledge wrote: > On Tue, Feb 06, 2018 at 01:48:19PM +0100, Vincent Lefevre wrote: > > On 2018-02-05 09:39:12 -0500, Greg Wooledge wrote: > > > Anyway, here's what I came up with: > > > > > > lastday() { > > > > > > date +%Y-%m-%d -d "$1 1 day ago + 1 month" > > > > > > } > > > > But the exact meaning of "month" seems undocumented, which may > > silently break in a future version (e.g. possibly as a consequence > > of a bug fix). So this is a good example of what you should *not* > > do. > > If I have the luxury of writing in a real language, then I prefer > to use that language's date/time libraries, if they exist and don't > suck. > > In bash, you get this, or you get to write your own date/time code > by hand. > > > This is completely crazy: > > > > Where is the logic behind that? > > As noted by someone else earlier in this thread, date recently learned > a --debug option (this is my first time hearing about it), so let's > try that: > > wooledg:~$ date --debug +%Y-%m-%d -d '2003-09-01 1 day ago + 1 month' > date: parsed date part: (Y-M-D) 2003-09-01 > date: parsed relative part: -1 day(s) > date: parsed relative part: +1 month(s) -1 day(s) > date: input timezone: -05:00 (set from system default) > date: warning: using midnight as starting time: 00:00:00 > date: starting date/time: '(Y-M-D) 2003-09-01 00:00:00 TZ=-05:00' > date: warning: when adding relative months/years, it is recommended to > specify the 15th of the months date: warning: when adding relative days, > it is recommended to specify 12:00pm date: after date adjustment (+0 > years, +1 months, -1 days), > date: new date/time = '(Y-M-D) 2003-09-30 00:00:00 TZ=-05:00' > date: '(Y-M-D) 2003-09-30 00:00:00 TZ=-05:00' = 1064894400 epoch-seconds > date: output timezone: -05:00 (set from system default) > date: final: 1064894400.000000000 (epoch-seconds) > date: final: (Y-M-D) 2003-09-30 04:00:00 (UTC0) > date: final: (Y-M-D) 2003-09-30 00:00:00 (output timezone TZ=-05:00) > 2003-09-30 > > So, it adds "a month" first, then subtracts "a day" second. I suppose > I could have rearranged the sub-arguments to make that clearer, but > honestly, I stopped tweaking it once I had a working invocation. It's > so fragile that I didn't want to change *anything* for fear of breaking > it. > > In some cases, I dealt with this stuff using a truly ancient version > of GNU date from the sh-utils package, before coreutils existed. Some > of the more modern invocations didn't work with the older program, so > I often had to find different tricks that would work. I don't know > whether this was one such case. It could have been.