Every point on Earth has a time zone for organized activity and coordination, established usually within the country or continent boundaries.
Lines of longitude connected at the North and South Poles determine global time zones.
But the North and South Poles, present at the Earth's extremes, lack conventional time zones.
All lines of longitude converge at the Poles, which places them in all the time zones simultaneously, a reason why it lacks conventional time zones.
Hence, polar explorers & scientists in Poles choose time zones for convenience on their own, including using their country's standard time or Greenwich Mean Time.
Greenwich Mean Time is the local mean time at the Royal Observatory in Greenwich, London, counted from midnight.