Definition of year (Julian) unit: = 365.25 d average. The basic calendar year has 365 days, but in fact the Earth needs a little more time around the sun. The Julian calendar compensated this with leap year (every fourth year had 366 days). According to the formula, a Julian year has 365 + 1/4 = 365.25 days. But even with this compensation, the astronomical year slip 0.78 days every 100 years into real seasons. Therefore, most countries have moved to the Gregorian calendar introduced in 1582.