Wikidata gained the functionality of adding dates as an assertion to an item. It is really nice because it is now possible to indicate when a person was born or died.
At this moment we are in the last week of the month of Ramadan, in the year 1434. When you check a calendar forthis month, you may notice that Ramadan is partly in the month of July and August 2013.
When you are adding assertions to Wikidata relating to Islam or Arabic history, you will find that many sources use the Islamic calendar and not the Gregorian dates. As a consequence you have to convert them to Gregorian before you can enter them into Wikidata.
To solve this Wikidata could accept Islamic dates. It just takes some programming. With a little bit of help from Google you can find PHP code like I did. It will probably provide basic functionality. I cannot tell.
The question is, is there someone who is willing to write the code in Wikidata so that it will provide support for Islamic dates? I am sure there will be all kind of issues to resolve.