date command help

TK_2008 · July 31, 2008, 1:48am

Hi
Todays date command said "last month is this month !"

$ cat /etc/redhat-release
Red Hat Enterprise Linux ES release 4 (Nahant Update 3)

$ date
Thu Jul 31 14:21:21 JST 2008

$ date +%Y-%m
2008-07

$ date --date "last month" +%Y-%m
2008-07

$ date --date 'last month'
Tue Jul 1 14:21:34 JST 2008

why ?

era · July 31, 2008, 2:14am

Quoting the date info page:

The fuzz in units can cause problems with relative items. For example, `2003-07-31 -1 month' might evaluate to 2003-07-01, because 2003-06-31 is an invalid date. To determine the previous month more reliably, you can ask for the month before the 15th of the current month. For example:
$ date -R
Thu, 31 Jul 2003 13:02:39 -0700
$ date --date='-1 month' +'Last month was %B?'
Last month was July?
$ date --date="$(date +%Y-%m-15) -1 month" +'Last month was %B!'
Last month was June!
Also, take care when manipulating dates around clock changes such as daylight saving leaps. In a few cases these have added or subtracted as much as 24 hours from the clock, so it is often wise to adopt universal time by setting the `TZ' environment variable to `UTC0' before embarking on calendrical calculations.

TK_2008 · July 31, 2008, 6:02am

Thanks era for your help.