Daylight saving time is something that only applies in some places. The purpose is to shift hours of daylight from early morning when most people are asleep into the evening when they are more likely to be awake to benefit from the daylight.
It doesn't apply close to the equator as there the day doesn't vary greatly in length and so there isn't much if any daylight prior to people waking up to move. It doesn't apply close to the poles because there the daylight soon extends into the evening without needing to borrow some from the early morning. Daylight saving time only applies in some of those places in between where the day varies enough in length to provide early morning daylight that can effectively be moved to the evening so as to provide more daylight when most people are awake.
That the offset changes depending on whether or not daylight saving applies is sufficient for us to determine whether they are currently on daylight saving time. Since where daylight saving time does apply, it applies for a period during summer and doesn't apply for a period in winter. The exact start and end dates vary from place to place and which part of the year is summer and which winter depends on which hemisphere you are in. What we can reasonably say based on this though is that if we pick two dates where one is in the middle of summer and the other in the middle of winter that they will be in different timezones if the place uses daylight saving time.
This article written by Stephen Chapman, Felgall Pty Ltd.