Windows asks users to choose from about 200 timezones, when there are actually only 25 GMT offsets. (see below)
Can I ask users in my app to chose from these alone? Is there any difference between timezones that have the same GMT offset? (eg: Mountain Standard Time vs. Mexico Standard Time 2)
-12.0,GMT -12:00,Eniwetok, Kwajalein
-11.0,GMT -11:00,Midway Island, Samoa
-10.0,GMT -10:00,Hawaii
-9.0,GMT -9:00,Alaska
-8.0,GMT -8:00,Pacific Time, US & Canada
-7.0,GMT -7:00,Mountain Time, US & Canada
-6.0,GMT -6:00,Central Time, US & Canada, Mexico City
-5.0,GMT -5:00,Eastern Time, US & Canada, Bogota, Lima
-4.0,GMT -4:00,Atlantic Time, Canada, Caracas, La Paz
-3.5,GMT -3:30,Newfoundland
-3.0,GMT -3:00,Brazil, Buenos Aires, Georgetown
-2.0,GMT -2:00,Mid-Atlantic
-1.0,GMT -1:00 hour,Azores, Cape Verde Islands
0.0,GMT,Western Europe Time, London, Lisbon, Casablanca
1.0,GMT +1:00 hour,Brussels, Copenhagen, Madrid, Paris
2.0,GMT +2:00,Kaliningrad, South Africa
3.0,GMT +3:00,Baghdad, Riyadh, Moscow, St. Petersburg
3.5,GMT +3:30,Tehran
4.0,GMT +4:00,Abu Dhabi, Muscat, Baku, Tbilisi
4.5,GMT +4:30,Kabul
5.0,GMT +5:00,Ekaterinburg, Islamabad, Karachi, Tashkent
5.5,GMT +5:30,Bombay, Calcutta, Madras, New Delhi
5.75,GMT +5:45,Kathmandu
6.0,GMT +6:00,Almaty, Dhaka, Colombo
7.0,GMT +7:00,Bangkok, Hanoi, Jakarta
8.0,GMT +8:00,Beijing, Perth, Singapore, Hong Kong
9.0,GMT +9:00,Tokyo, Seoul, Osaka, Sapporo, Yakutsk
9.5,GMT +9:30,Adelaide, Darwin
10.0,GMT +10:00,Eastern Australia, Guam, Vladivostok
11.0,GMT +11:00,Magadan, Solomon Islands, New Caledonia
12.0,GMT +12:00,Auckland, Wellington, Fiji, Kamchatka
In general no, you can’t. Time zone differs not only by default GMT offset, but also by DST (daylight saving time). So two TZ can have same offset, but one with DST and other without. Even TZ with same offset and both with DST can differ in DST start and end time.