[tz] Calculating dates of Ramadan

[Ian Abbott]

On 2012-05-16 10:28, Tim Thornton wrote:
> Regarding calculating Ramadan:
> The Islamic calendar is lunar, not solar, and is about 11 or 12 days shorter
> than the "normal" Gregorian calendar.
> Months (including Ramadan) start when the first crescent of a new moon is
> sighted. This varies according to the atmospheric conditions and the exact
> positions of earth, sun and moon, but can be calculated to within about a
> day for any given location, which is good enough for tz data in the absence
> of any further specific information for a location.
> There is also an ongoing debate in Islam on whether the calculated date is
> good enough, or whether they should stick with past practice of having
> official observers watching the sky to announce the start of the month.
> What cannot be done is to say that it always falls between a given pair of
> Gregorian calendar dates, due to its shifting.
>
> Tim

In practice, how much does the observed (as in astronomical) date differ 
from the calculated date?  Does it ever start _before_ the calculated date?

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