Baktuns - periods of 400 years - are counted on a base-13 cycle. On December 20, 2012, the Maya Long Count Date was 12.19.19.19.19. When one day was then added, the entire calendar reset to 0. The thirteenth Baktun since the beginning of Maya time therefore came to an end on December 21, 2012. This of course led to much speculation about dramatic changes: some predictions for the end of the Maya Long Count Calendar included the end of the world, a new age of consciousness, a reversal of the Earth's magnetic poles, the arrival of the Messiah, etc. Needless to say, none of those things happened. In any event, historical Maya records do not indicate that they gave much thought to what would happen at the end of the calendar.
Source: https://www.thoughtco.com/the-maya-calendar-2136178
