A calendar year with an additional day or month is added to maintain synchronization with the astronomical or seasonal year.This correction helps correct the drift between a civilization's dating system and the physical properties of the Solar System, as astronomical events and seasons do not repeat in a whole number of days.The Hebrew calendar adds Adar Aleph seven times every 19 years to keep its calendar year from drifting through the seasons.The Gregorian calendar adjusts for this error by adding an extra leap day in each year that is a multiple of 4.