I have never worked with Hijri dates before. So please take a job below with a pinch of salt. But I guess this is one way in which Gregorian dates could be converted to Hijri format through code.
static void testHijriDate(Args _args)
{
TransDate dt;
int HijriDt;
int HijriMonth;
str Yr;
CalendarConverter CalendarConverter;
;
dt = 01\01\2011;
CalendarConverter = new CalendarConverter(PreferredCalendar::Hijri);
HijriDt = CalendarConverter.dayofmth(dt);
HijriMonth = CalendarConverter.mthofyr(dt);
Yr = CalendarConverter.yearStr(dt);
info(strFmt(‘Hijri day %1 - month %2 - Year %3’, HijriDt, HijriMonth, Yr));
}