Setting NSCalendarDate Time

NSCalendarDates are immutable, so there's no way to change them after they've been initialized. Two NSCalendar dates are necessary, as follows:

Code:

NSCalendarDate *currentDate = [NSCalendarDate currentDate];
NSCalendarDate *todayMidnight = [NSCalendarDate
    dateWithYear: [currentDate yearOfCommonEra]
    month: [currentDate monthOfYear]
    day: [currentDate dayOfMonth]
    hour: 0
    minute: 0
    second: 0
    timeZone: nil];

Remember to retain as necessary.