If DST is in use, then specific rules define the point at which the offset changes and the amount by which it changes. The city chosen is not the only city in which the zone applies, but rather a representative city for the region. An offset from GMT, either positive or negative.
Offsets range from approximately minus half a day to plus half a day.
The precise date and time during the year when DST begins. In the first half of the year it's in the northern hemisphere, and in the second half of the year it's in the southern hemisphere. The precise date and time during the year when DST ends. In the first half of the year it's in the southern hemisphere, and in the second half of the year it's in the northern hemisphere. The amount by which the GMT offset changes when DST is in effect.