title: Add a Jalali month to a Jalali Date
body: <p>I have a date in Jalali (Persian system of Date) which I need to be forwarded 1 month. The problem is that when I add 1 month using <code>myDateTime.AddMonths(1)</code> my date forwards by 1 Gregorian month, whereas I want a Jalali month.</p>
  
  <p>For example if my Date (Format: yyyy-MM-dd) is <strong>2013-02-28</strong> Gregorian and <strong>1391/12/10</strong> Jalali, and I add one month it will be <strong>2013-03-28</strong> Gregorian (that is alright) and <strong>1392/01/08</strong> (that is wrong and should be <em>1392/01/10</em> ).</p>
  
  <p>Having said all the story above, is there a way I can Add one month to a date based on my culture or region or something?</p>