UPDATE

使用答案中的信息,我想出了这种方法,我经常使用：

// Generate a random date sometime between now and n days before day.
// Also,generate a random time to go with the day while we are at it.
- (NSDate *) generateRandomDateWithinDaysBeforeToday:(NSInteger)days
{
    int r1 = arc4random_uniform(days);
    int r2 = arc4random_uniform(23);
    int r3 = arc4random_uniform(59);
 
    NSDate *today = [NSDate new];
    NSCalendar *gregorian = 
             [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
 
    NSDateComponents *offsetComponents = [NSDateComponents new];
    [offsetComponents setDay:(r1*-1)];
    [offsetComponents setHour:r2];
    [offsetComponents setMinute:r3];
 
    NSDate *rndDate1 = [gregorian dateByAddingComponents:offsetComponents 
                                                  toDate:today options:0];
 
    return rndDate1;
}