[release-branch.go1.15] time: correct unusual extension string cases
This fixes two uncommon cases.
First, the tzdata code permits timezone offsets up to 24 * 7, although
the POSIX TZ parsing does not. The tzdata code uses this to specify a
day of week in some cases.
Second, we incorrectly rejected a negative time offset for when a time
zone change comes into effect.
For #44385
Fixes #44617
Change-Id: I5f2efc1d385e9bfa974a0de3fa81e7a94b827602
Reviewed-on: https://go-review.googlesource.com/c/go/+/296392
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
(cherry picked from commit
d9fd38e68ba00a51c2c7363150688d0e7687ef84)
Reviewed-on: https://go-review.googlesource.com/c/go/+/297229