How to set time on NSDate?

This works great as an NSDate category.

/** Returns a new NSDate object with the time set to the indicated hour, 
  * minute, and second.
  * @param hour The hour to use in the new date.
  * @param minute The number of minutes to use in the new date.
  * @param second The number of seconds to use in the new date.
  */
-(NSDate *) dateWithHour:(NSInteger)hour 
                  minute:(NSInteger)minute 
                  second:(NSInteger)second 
{
   NSCalendar *calendar = [NSCalendar currentCalendar];
   NSDateComponents *components = [calendar components: NSYearCalendarUnit|
                                                         NSMonthCalendarUnit|
                                                         NSDayCalendarUnit
                                               fromDate:self];
    [components setHour:hour];
    [components setMinute:minute];
    [components setSecond:second];
    NSDate *newDate = [calendar dateFromComponents:components];
    return newDate;
}

With the above category, if you have an existing date you want to change the time on, you do so like this:

NSDate *newDate = [someDate dateWithHour:10 minute:30 second:00];

If, however, you are trying to add or subtract hours from an existing date, a category method to do that is also straightforward:

/** Returns a new date with the given number of hours added or subtracted.
  * @param hours The number of hours to add or subtract from the date.
  */
-(NSDate*)dateByAddingHours:(NSInteger)hours
{
    NSDateComponents *components = [[NSDateComponents alloc] init];
    [components setHour:hours];

    return [[NSCalendar currentCalendar] 
               dateByAddingComponents:components toDate:self options:0];
}

Your approach should work fine. I needed a solution for this type problem (setting the individual date components) and the following code works as expected for me. My situation: I wanted to create a date object that used the current date but had the time set to a value that was passed in as a string.

NSString *string = @"7:00";
NSLocale *locale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US_POSIX"];
NSDateFormatter *timeOnlyFormatter = [[NSDateFormatter alloc] init];
[timeOnlyFormatter setLocale:locale];
[timeOnlyFormatter setDateFormat:@"h:mm"];

NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
NSDate *today = [NSDate date];
NSDateComponents *todayComps = [calendar components:(NSDayCalendarUnit | NSMonthCalendarUnit | NSYearCalendarUnit) fromDate:today];

NSDateComponents *comps = [calendar components:(NSHourCalendarUnit | NSMinuteCalendarUnit) fromDate:[timeOnlyFormatter dateFromString:string]];
comps.day = todayComps.day;
comps.month = todayComps.month;
comps.year = todayComps.year;
NSDate *date = [calendar dateFromComponents:comps];
[calendar release];
[timeOnlyFormatter release];
[locale release];

One thing to note is that you really have to pay attention to time zones when you are judging whether a time appears to be accurate. For example, in my app, when you stop at a breakpoint, you will see the time in GMT (so it looks different than the input time, which is in my local time), but when the time is actually displayed on screen in the app, it is being formatted to display in the local timezone. You may need to take this into consideration to determine whether the result is actually different from what you would expect.

If this does not help, can you elaborate on "not giving the desired result"? What result is it giving and how does that compare to what you expected?

is Swift2

extension NSDate {
    func dateWithHour (hour: Int, minute:Int, second:Int) ->NSDate?{

        let calendar = NSCalendar.currentCalendar(),
            components = calendar.components([.Day,.Month,.Year], fromDate: self)
        components.hour = hour;
        components.minute = minute;
        components.second = second;

        return calendar.dateFromComponents(components)
    }
}