The argument 
to CalendarChange converts between Western (Gregorian) dates and Islamic dates. This computes the Islamic date of January 1, 2000:
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The Islamic calendar has the following months:
The above date is therefore the 24th of Ramadan, 1420.
The Western date for the beginning of Ramadan of the Islamic year 1420:
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The Islamic year has fewer days than the Western year:
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The Islamic calendar is purely lunar (time of month corresponds to phase of the moon) but does not keep track of seasons, since there is no leap month. The Islamic calendar moves backward with respect to the Western calendar; for example, the start of Ramadan in successive years:
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| Out[3]= |  |
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, you first need to load the Calendar Package using 
.
is a value for the Calendar option, and can be used as an argument to CalendarChange. 
Julian, or 
in the 
calendar (one date given for the Hegira).
