MS Calendar Control use day,month and year properties values, so if you want the calendar value you can use this on calendar.click() event

CTOD(ALLT(STR(THIS.DAY))+'/'+ALLT(STR(THIS.MONTH))+'/'+ALLT(STR(THIS.YEAR)))

OR

to make this calendar shows some date,
LOCAL nDate
nDate={05/02/99}
thisform.calendar.DAY=INT(DAY(nDate))
thisform.calendar.MONTH=INT(MONTH(nDate))
thisform.calendar.YEAR=INT(YEAR(nDate))

HTH