The solar calendar is based on the earth's period of revolution around the sun, so a solar calendar year approximates a tropical year.

The widely used Gregorian Calendar is a solar calendar. According to the calendar, each year consists of 365 days (in most cases) divided into 12 months, with each month having the same number of days for every year. The length of the Gregorian Calendar was only 26 seconds different from that of a tropical year, meaning a difference of 1 day every 3,300 years.

However, since the length of a tropical year in days is not a whole number, to prevent the calendar from going out of tune with the movement of the sun, the calendar itself has a special feature. Certain years of the calendar have an extra day which is inserted to the second month, and are called leap years. The determination of a leap year under the Gregorian Calendar follows a simple algorithm: if the year can be divided by 4 and not by 100, or if it can be divided by 400, then it will be a leap year.

The Gregorian Calendar can be traced back to the ancient Egypt, where the first solar calendar in the world was invented based on the observations of the Sirius and the sun. So the calendar is also called Sirius Calendar. Later, ancient Romans mastered the calendar. Afterwards, the calendar spread throughout the world with the expansion of the Roman Empire and the rising of Christianity.