how do I set an existing NSDate's time?

Question

how do I set an existing NSDate's time?

That is I have a date (say current date/time), but then want to set this to a specific time (e.g. 11.23am) say. What is the quickest way to do this in objective C?

I'm looking at NSCalendar and see methods such as dateByAddingComponents:toDate:options:, and dateFromComponents:, but these don't seem to quite hit the mark for what I need here. For example:

1. dateFromComponents: - I would have to work out all components for this to work which seems like overkill
2. dateByAddingComponents:toDate:options: - I don't want to ADD in my case but rather "set" the time.

Answer 1

NSDate *date = [NSDate date];
NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier: NSGregorianCalendar];
NSDateComponents *components = [gregorian components: NSUIntegerMax fromDate: date];
[components setHour: 7];
[components setMinute: 59];
[components setSecond: 17];

NSDate *newDate = [gregorian dateFromComponents: components];

I use NSUIntegerMax instead of specific OR-combined bit masks because it's easier, but if you want to specify which data components to receive, be my guest.

Answer 2

iOS 7 Note: I'm putting this in a few of these questions because use of NSUIntegerMax no longer seems to work on iOS7, and using it caused me to chase my tail for about 3 hours. I just discovered that when I use NSUIntegerMax, NSDateComponentsignores set year. So for example this DOES NOT change the year:

NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
NSDateComponents *comps = [gregorian components:NSUIntegerMax fromDate:date];
[comps setYear:year];
return [gregorian dateFromComponents:comps];

Whereas this DOES:

NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
NSUInteger timeComps = (NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit | NSHourCalendarUnit | NSMinuteCalendarUnit | NSSecondCalendarUnit | NSTimeZoneCalendarUnit);
NSDateComponents *comps = [gregorian components:timeComps fromDate:date];
[comps setYear:year];
return [gregorian dateFromComponents:comps];

It may be an iOS7 bug or it may be that using NSUIntegerMax working was a fluke which no longer works. I will file a bug report and get a definitive answer.

Regardless, if you want to avoid unexpected consequences specify ALL the components else you might be chasing your tail!

Answer 3

Swift 2.0 version of how you'd set a given date to 8am

extension NSDate {

func setTo8AM() -> NSDate {
    let calendar = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)!
    let components = calendar.components(([.Day, .Month, .Year]), fromDate: self)
    components.hour = 8
    return calendar.dateFromComponents(components)!
}

}

Answer 4

Apply this to set the Time from an another date to an existing Date

NSDate *today = [NSDate date]; // from which i need only "Year" "Month" "Date"

NSCalendar *calendar = [NSCalendar currentCalendar]
NSDateComponents *dateComponents = [calendar components:( NSYearCalendarUnit | NSMonthCalendarUnit |  NSDayCalendarUnit ) fromDate:today];

NSDate *timeDate = datePicker.date; // from which i need only "Hours" "Minutes" and "Seconds"

NSDateComponents *timeComponents = [calendar components:( NSHourCalendarUnit | NSMinuteCalendarUnit | NSSecondCalendarUnit ) fromDate:timeDate];

            [dateComponents setHour:[timeComponents hour]];
            [dateComponents setMinute:[timeComponents minute]];
            [dateComponents setSecond:[timeComponents second]];

NSDate *newDate = [calendar dateFromComponents:dateComponents]; // New Date with My "Year" "Month" "Date" "Hours" "Minutes" and "Seconds" 

            NSLog(@"New Date: %@",newDate);