On Sun, Apr 25, 2004 at 05:36:59PM +0200, Andreas Johansson wrote:
> It seems to me it would be inconvenient for trade and diplomacy if you
> could not know what date it was in the next city.

But this situation was common in ancient times when adjacent city-states
might not even use the same basic calendar system.  As long as a
correspondence between the two systems can be established in, er,
correspondence, each side can keep records in its own system and it's
only a minor inconvenience.  When both sides use the same system but
disagree on the particulars, it can be confusing, but in the case of the
Islamic calendar, as I said, it never amounted to more than a day or so
and was tantamount to time zone differences.

The Islamic situation is, in terms of magnitude, much less of a problem
than, for instance, that in Europe during the century and a half when
most of the Continent had adopted the Gregorian reform but England was
still on the Julian calendar.  An Englishman corresponding to a
Frenchman would write e.g. February 3, 1678 for the date his
correspondent regarded as February 13, 1679.

There is today still no central authority for observation of the new
Islamic month, by the way, but it's less important since the calendar is
no longer used for civil purposes, and for religious purposes the
important date is the one local to the location of the observance (the
annual Haaj pilgrimage is governed by the calendar according to the
authorities in Mecca, for instance, no matter the date in a given
pilgrim's locale of origin).