Default AR culture in .Net using Hijri calendar, all others cultures use Gregorian
You have two way how to fix that:
1) Use AR language with different culture. For example Egypt, UAE. You can find more information here: http://stackoverflow.com/questions/359986/how-to-display-arabic-dates-in-the-gregorian-calendar
2) If you can't use AR with different culture, you need to edit widgets templates for every widget. I posted answer to similar question here: http://www.sitefinity.com/developer-network/forums/developing-with-sitefinity-/arabic-site-news-listing-to-show-date-in-gregorian
Very appreciated .
AR is equal to AR-SA (details is here: http://www.csharp-examples.net/culture-names/)
In your case - yes, it is way faster to fix widget templates