Count Down Timer in iPhone - iphone

Question

I have two time.. one fetched directly as string(#"21:00") and other is the current time. I want to display a count down timer showing how much time left from now to reach 21:00.
For eg: the label i use should display "You have 3hrs and 30 minutes left.." if the current time is 17:30.
thanks..

Answer 1

OK have completely revised my answer, and have created a complete solution to the problem, with fully tested sample code available on github.
Enjoy :)

Answer 2

Something like this should do it. This will give you the remaining time in seconds. Then you just need to to standard timer stuff as indicated in other answers.
//assumption: targetTime is after now
NSString *targetTime = #"21:00";
//split our time into components
NSArray *timeSplit = [targetTime componentsSeparatedByString:#":"];
NSUInteger hours = [[timeSplit objectAtIndex:0] intValue];
NSUInteger minutes = [[timeSplit objectAtIndex:1] intValue];
NSDate *now = [NSDate date];
//split now into year month day components
NSCalendar *currentCalendar = [NSCalendar currentCalendar];
NSDateComponents *dateComponents = [currentCalendar components:NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit fromDate:now];
//set our time components from above
[dateComponents setHour:hours];
[dateComponents setMinute:minutes];
NSDate *targetDate = [currentCalendar dateFromComponents:dateComponents];
//ensure target is after now
if ([targetDate timeIntervalSinceDate:now] < 0)
{
NSDateComponents *day = [[NSDateComponents alloc] init];
[day setDay:1];
targetDate = [currentCalendar dateByAddingComponents:day toDate:targetDate options:0];
}
NSTimeInterval timeRemaining = [targetDate timeIntervalSinceDate:now];

Answer 3

You create a NSTimer that fires every second:
NSTimer *timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target:self selector:#selector(tickClock) userInfo:nil repeats:YES];
When it fires, you enter a method:
- (void)tickClock;
in there, you compare the date you have against the current date [NSData currentDate];
Like
NSTimeInterval distanceBetweenDates = [currentDate timeIntervalSinceDate:yourDate];
where distanceBetweenDates is specified in seconds.
To create a date from your string create a NSDate accordingly
NSInteger year = 2011;
NSInteger month = 8;
NSInteger day = 26;
NSInteger hour = 21;
NSInteger minute = 0;
NSInteger second = 0;
NSCalendar *calendar = [NSCalendar currentCalendar];
NSDateComponents *components = [[NSDateComponents alloc] init];
[components setYear:year];
[components setMonth:month];
[components setDay:day];
[components setHour:hour];
[components setMinute:minute];
[components setSecond:second];
NSDate *date = [calendar dateFromComponents:components];
[components release];