Solar calendar, also known as the Gregorian calendar, is the calendar used worldwide. The dates on the calendar that are written in Arabic numerals are solar calendar dates. For example, September 9, 1999, is a solar calendar date.

The solar calendar, in astronomical terms, refers to a calendar primarily arranged according to the annual motion of the sun. It has about 365 days in a year. The solar calendar is designed based on the cycle of the sun's direct rays, with an average year length of a tropical year. There are two types of years: common years and leap years, which differ by only one day. Therefore, some years have 365 days, and some have 366 days.