bin/62134: strange date -v results

[Edwin Groothuis]

The following reply was made to PR bin/62134; it has been noted by GNATS.

From: Edwin Groothuis <edwin at mavetju.org>
To: Danijel Tasov <dt at vosat.de>
Cc: FreeBSD-gnats-submit at FreeBSD.org
Subject: Re: bin/62134: strange date -v results
Date: Sat, 31 Jan 2004 13:30:43 +1100

 man date says:
 	Adjust (i.e., take the current date and display the result
 	of the adjustment; not actually set the date) the second,
 	minute, hour, month day, week day, month or year according
 	to val.  If val is preceded with a plus or minus sign, the
 	date is adjusted forwards or backwards according to the
 	remaining string, otherwise the relevant part of the date
 	is set.  The date can be adjusted as many times as required
 	using these flags.  Flags are processed in the order given.
 
 	[...]
 
 	When the date is adjusted to a specific value that doesn't
 	actually exist (for example March 26, 1:30 BST 2000 in the
 	Europe/London timezone), the date will be silently adjusted
 	forwards in units of one hour until it reaches a valid time.
 	When the date is adjusted to a specific value that occurs
 	twice (for example October 29, 1:30 2000), the resulting
 	timezone will be set so that the date matches the earlier
 	of the two times.
 
 So what you're doing is, on januari 30th or 31st:
 
     for i in 1 2 3; do date -v2004y -v${i}m -v17d +%Y-%m-%d; done
 
 Loop i=1:
     step 1. current date is 2004-01-31
     step 2. -v2004y: set current year to 2004, date will become 2004-01-31
     step 3. -v1m: set current month to 1, date will become 2004-01-31
     step 4. -v17d: set current day to 17, date will become 2004-01-17
 
 Loop i=2:
     step 1. current date is 2004-01-31
     step 2. -v2004y: set current year to 2004, date will become 2004-01-31
     step 3. -v2m: set current month to 2, date will become 2004-02-31
 	    Unfortunatly this date doesn't exist, so it takes the next
 	    best value and date will become 2004-03-01.
     step 4. -v17d: set current day to 17, date will become 2004-03-17
 
 Loop i=3:
     step 1. current date is 2004-01-31
     step 2. -v2004y: set current year to 2004, date will become 2004-01-31
     step 3. -v3m: set current month to 2, date will become 2004-03-31
     step 4. -v17d: set current day to 17, date will become 2004-03-17
 
 Same problem will happen when you add the 4th month to the list.
 
 
 So the right syntax would be:
     for i in 1 2 3; do date -v17d -v${i}m -v2004y +%Y-%m-%d; done
 
 -- 
 Edwin Groothuis      |            Personal website: http://www.mavetju.org
 edwin at mavetju.org    |          Weblog: http://weblog.barnet.com.au/edwin/