one: the calendar

The Hebrew calendar is used to determine Jewish holidays, the anniversary of a person's death, and Torah portion readings for synagogue services. It is a 12-month calendar of either 354 or 355 days. The length of its years roughly coincides with how long it takes for the Earth to revolve around the Sun; its months roughly coincide with the Moon's cycle. Because there is about an 11-day difference between 12 lunar months and 1 solar year, the Hebrew calendar adds an extra month 7 times over the course of 19 years.

The Christian/Gregorian calendar is accepted by most Western nations. It is named for Pope Gregory XIII. On February 24, 1582, he decreed by papal bull a modification of the calendar introduced by Julius Caesar in 46 BCE (the Julian calendar). The Gregorian calendar is a 12-month solar calendar based on the number of days it takes for the Earth to make one revolution around the Sun (365.242). Because the Julian calendar caused a seasonal drift, the Gregorian calendar removed 10 days as a one-time correction (October 4 was immediately followed by October 15) and changed the rule for when leap years occur. The year number is calculated from the traditional birth year of Jesus Christ. This was once referred to as anno Domini (Latin for "the year of our Lord") or AD, and is now called Christian or Common Era, or CE.

The Muslim calendar is a 12-month lunar calendar of 354 days. The start of a new month is traditionally determined by human observation of the moon's waxing crescent—first sliver of moon visible after the darkness of the new moon. As such, the calendar is dependant on factors like the weather and the location of the observer. The year number is calculated from the start of Muhammad's hijra (emigration) from Mecca to Medina, which took place in 622 CE, or 1 AH. The designation AH year stands for anno Hegirae, Latin for "the year of the Hijra."