There are no permanent residents on the continent. But anywhere from 1000 to 5000 people, most of whom are scientists, reside in various research stations scattered across the continent throughout the year. So keeping watch of the time is very important.
Time zone is determined by the line of longitude. India for instance, is located several time-zones west of Malaysia, and hence the time in Malaysia is several hours ahead of India. The difference in time is determined by the difference in the longitude line, with each degree divided into 60 minutes.
Antarctica sits in all time zones, so adhering to the rule above means that... erm...
For practical purposes time zones are usually based on territorial claims, however many stations use the time of the country they are owned by or the time zone of their supply base (e.g. McMurdo Station and Amundsen–Scott South Pole Station use New Zealand time due to their main supply base being Christchurch, New Zealand).--wikipedia