How Do I write a Timer in Objective-C?

Don't use NSTimer that way. NSTimer is normally used to fire a selector at some time interval. It isn't high precision and isn't suited to what you want to do.

What you want is a High resolution timer class (using NSDate):

Output:

Total time was: 0.002027 millisecondsTotal time was: 0.000002 secondsTotal time was: 0.000000 minutes

Main:

Timer *timer = [[Timer alloc] init];[timer startTimer];// Do some work[timer stopTimer];NSLog(@"Total time was: %lf milliseconds", [timer timeElapsedInMilliseconds]);  NSLog(@"Total time was: %lf seconds", [timer timeElapsedInSeconds]);NSLog(@"Total time was: %lf minutes", [timer timeElapsedInMinutes]);

Edit: Added methods for -timeElapsedInMilliseconds and -timeElapsedInMinutes

Timer.h:

#import <Foundation/Foundation.h>@interface Timer : NSObject {    NSDate *start;    NSDate *end;}- (void) startTimer;- (void) stopTimer;- (double) timeElapsedInSeconds;- (double) timeElapsedInMilliseconds;- (double) timeElapsedInMinutes;@end

Timer.m

#import "Timer.h"@implementation Timer- (id) init {    self = [super init];    if (self != nil) {        start = nil;        end = nil;    }    return self;}- (void) startTimer {    start = [NSDate date];}- (void) stopTimer {    end = [NSDate date];}- (double) timeElapsedInSeconds {    return [end timeIntervalSinceDate:start];}- (double) timeElapsedInMilliseconds {    return [self timeElapsedInSeconds] * 1000.0f;}- (double) timeElapsedInMinutes {    return [self timeElapsedInSeconds] / 60.0f;}@end