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number of days between two dates may be calculated. For example, "25 August 2023" is ten days after "15 August 2023". The date of a particular event depends on the observed time zone. For example, the air attack on Pearl Harbor that began at 7:48 a.m. Hawaiian time on 7 December 1941 took place at 3:18 a.m. Japan Standard Time, 8 December in Japan. A particular day may be assigned a different nominal date according to the calendar used, so an identifying suffix may be needed where ambiguity may arise.[a] The Gregorian calendar is the world´s most widely used civil calendar, and is designated (in English) as AD or CE. Many cultures use religious or regnal calendars such as the Gregorian (Western Christendom, AD), Hebrew calendar (Judaism, AM), the Hijri calendars (Islam, AH), Julian calendar (Eastern Christendom, AD) or any other of the many calendars used around the world. In most calendar systems, the date consists of three parts: the (numbered) day of the month, the month, and the (numbered) year. There may also be additional parts, such as the day of the week. Years are usually counted from a particular starting point, usually called the epoch, with era referring to the span of time since that epoch.[b]