Depends on the scale of your application. If it's truely global, then timezones change often. I think Russia tends to change it's timezones every 5 years or so
Actually, the Russian time zone change only changes the rules of those time zoned, not the geographical extent of them. Since we just return a tz id, users can just use an updated tz database (any up-to-date Linux system has this) to get the gmt offset at a particular time.
Russia is even worse than that. One year they "cancel winter time", essentially moving to daylight saving time for the whole year. And now parliament is discussing reversing that.
Depends on the scale of your application. If it's truely global, then timezones change often. I think Russia tends to change it's timezones every 5 years or so