December solstice

The December solstice, also known as the southern solstice, is the solstice that occurs each December – typically on 21 December, but may vary by one day in either direction according to the Gregorian calendar. In the Northern Hemisphere, the December solstice is the winter solstice, whilst in the Southern Hemisphere it is the summer solstice.
Solar year.
The December solstice solar year is the solar year based on the December solstice. It is thus the length of time between adjacent December solstices.
The length of the December solstice year has been relatively stable between 6000 BC and 2000 at 49:30 (minutes:seconds) to 50:00 in excess of 365 days and 5 hours. After 2000, it is getting shorter. In 4000, the excess time will be 48:52 and, in 10000, 46:45.