private static int ToDateTimeMonth( Calendar calendar, DateTime value )
{
Contract.Requires( calendar != null );
Contract.Ensures( Contract.Result<int>() > 0 );
var epoch = calendar.FirstMonthOfYear();
var month = calendar.GetMonth( value );
var actual = ( epoch + month ) - 1;
return actual > 12 ? ( actual - 12 ) : actual;
}