Sort NSArray of date strings or objects

If I have an NSMutableArray of objects with a field “beginDate” of type NSDate I am using an NSSortDescriptor as below:

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"beginDate" ascending:TRUE];
[myMutableArray sortUsingDescriptors:[NSArray arrayWithObject:sortDescriptor]];
[sortDescriptor release];