Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1
I think this implementation is heading in the wrong direction. To determine
whether a time zone string FOO is valid, a program should call tzalloc (FOO) and
sees whether that yields NULL. And if tzalloc doesn't work that way now, we
should fix tzalloc so it does. Once that happens, 'date' can simply call tzalloc
(getenv ("TZ")) to see whether the time zone setting is OK.