bug#11125: date command calculations are not consistent

[Eric Blake]

On 03/29/2012 12:34 PM, Thomas R. Schaefer wrote:

> In this case date is calculating with 24 hours days regardless of crossing a 
> DST boundary..
> 
> address@hidden ~]# date -d "Thu Mar 22 00:00:00 CDT 2012 - 21 days"
> Wed Feb 29 23:00:00 CST 2012

This started from one fixed point in time, relative to the 'CDT' time
zone, and subtracted 21 * 24 hours.

> 
> In this case date does take DST into account in a relative date operation..
> 
> address@hidden ~]# date -d "last Thursday - 21 days"
> Thu Mar  1 00:00:00 CST 2012

But notice what date -d "last Thursday" is:

$ TZ=CST date -d 'last Thursday CST'
Thu Mar 22 00:00:00 CST 2012

It's relative to the 'CST' timezone, which is an hour different from the
CDT timezone.

> 
> If the date command where being consistent in following the consensus that 
> "relative date operations add or subtract in multiples of 24 hours, without 
> regards to daylight savings boundaries" then both of the above date commands 
> would return Wed Feb 29 23:00:00 CST 2012.

Only if you start from the same point in time in both commands, which
you didn't.

-- 
Eric Blake   address@hidden    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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