http://www.timeanddate.com/calendar/moonphases.html?year=2011&n=015 Jul-13 Aug-12 Sep-12 Oct
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In this DURATION there are 12 full moons, whicih is the minimum number. That is, there has to be minimum 12 full moons in the time span between the two appearances of the same full moon so that you can call it a DURATION. 12 is a must. If it is fewer than 12 like 11, 10, 9..., you can not call it a duration and you can not use it to date, say, your bills.
On the other hand, there surely are 13 full moons in some durations, which proves 12 is minimum but not the maximum number. 13 is also God's truth. A good thing too, because this is the way full moon durations are suited to the solar year.
As for the time of the abstinence, God says: whoever of you witness the scorrching full moon shall abstain in it
(2:185) - فمن شهد منكم الشهر فليصمه
There need to be no question about it: fasting (abstinence) starts the day following the evening when the scorching full moon rises. 16 July
this year, 4 July