According to the Guinness World Records organization, the Jewish Calendar, which has been in use since the 9th century C.E., is the oldest calendar still in popular use. This calendar is used to determine the dates of Jewish holidays such as Rosh Hashanah and Yom Kippur.
This calendar is also known as the Hebrew calendar. There are examples of calendars that are older than the Hebrew calendar, but these are obsolete. For example, in 2013, archaeologists discovered what is described as the oldest calendar known to man in Scotland; this primitive calendar was not in use at the time of this discovery.