Areas south of the Antarctic Circle experience extreme day-night cycles near the times of the June and December solstices, making it difficult to determine which time zone would be most appropriate. For practical purposes time zones are usually based on territorial claims; however, the time zone of their supply base is often utilised (e.g., McMurdo Station and Amundsen–Scott South Pole Station use New Zealand time due to their main supply base being Christchurch, New Zealand). In most areas south of 80 degrees latitude, Coordinated Universal Time (UTC) is assumed despite the limited presence of clocks.

https://en.wikipedia.org/wiki/Time_in_Antarctica