Mean tropical year

This constant, the tropical year, is defined as the period of time for the mean ecliptic longitude of the Sun to increase by 360 degrees. Since the Sun's ecliptic longitude is measured with respect to the equinox, the tropical year comprises a complete cycle of the seasons; because of the biological and socio-economic importance of the seasons, the tropical year is the basis of most calendars. The modern definition of mean tropical year differs from the actual time between passages of e.g. the northward equinox for several reasons explained below. Because of the Earth's axial precession, this year is about 20 minutes shorter than the sidereal year. The mean tropical year is approximately 365 days, 5 hours, 48 minutes, 45 seconds^[5] (= 365.24219 days of 86400 SI seconds), averaged over equinoxes and solstices, with the northern equinox tropical year (used for the Gregorian calendar) currently being 365.2424 days.¹ 

1. ^ http://en.wikipedia.org/wiki/Year#Sidereal.2C_tropical.2C_and_anomalistic_years