bug#36383: date command processes timezone differently when doing math

[Brian Woods]

When doing a math operation to a date command it appear to process the
timezone differently.

The system is Ubuntu 18.04.2 LTS.
Versions: coreutils/bionic,now 8.28-1ubuntu1 amd64 [installed]

#echo $datNow
2019-06-25 15:21:34

#date -d "$datNow " "+%Y-%m-%d %H:%M:%S" --debug
date: parsed date part: (Y-M-D) 2019-06-25
date: parsed time part: 15:21:34
date: input timezone: system default
date: using specified time as starting value: '15:21:34'
date: starting date/time: '(Y-M-D) 2019-06-25 15:21:34'
date: '(Y-M-D) 2019-06-25 15:21:34' = 1561501294 epoch-seconds
date: timezone: system default
date: final: 1561501294.000000000 (epoch-seconds)
date: final: (Y-M-D) 2019-06-25 22:21:34 (UTC)
date: final: (Y-M-D) 2019-06-25 15:21:34 (UTC-07)
2019-06-25 15:21:34

#date -d "$datNow + 1 minute" "+%Y-%m-%d %H:%M:%S" --debug
date: parsed date part: (Y-M-D) 2019-06-25
date: parsed time part: 15:21:34 UTC+01
date: parsed relative part: +1 minutes
date: input timezone: parsed date/time string (+01)
date: using specified time as starting value: '15:21:34'
date: starting date/time: '(Y-M-D) 2019-06-25 15:21:34 TZ=+01'
date: '(Y-M-D) 2019-06-25 15:21:34 TZ=+01' = 1561472494 epoch-seconds
date: after time adjustment (+0 hours, +1 minutes, +0 seconds, +0 ns),
date:     new time = 1561472554 epoch-seconds
date: timezone: system default
date: final: 1561472554.000000000 (epoch-seconds)
date: final: (Y-M-D) 2019-06-25 14:22:34 (UTC)
date: final: (Y-M-D) 2019-06-25 07:22:34 (UTC-07)