There are a few different definitions for the year, one being the tropical year and another being the solar year. The solar year is the time it takes the earth to make one complete orbit around the sun. The tropical year is the average of the time between March equinoxes, and is about 10 minutes shorter than the solar year. The calendar is tied to the tropical year, which is why the seasons don't drift over the centuries, but the earth's perihelion and apohelion do, since those are tied to the solar year and the calendar isn't.
There are a few different definitions for the year, one being the tropical year and another being the solar year. The solar year is the time it takes the earth to make one complete orbit around the sun. The tropical year is the average of the time between March equinoxes, and is about 10 minutes shorter than the solar year. The calendar is tied to the tropical year, which is why the seasons don't drift over the centuries, but the earth's perihelion and apohelion do, since those are tied to the solar year and the calendar isn't.