AKServerTime (Obj-C/Cocoa). Keeps track of server time, adjusting for time zone/time sync diffs and accounting for latency.

AKServerTime.h

// AKServerTime
// By Henrik Nyh <http://henrik.nyh.se> 2010-03-14 under the MIT license.
//
// Keeps track of the current server time, adjusting for differences in
// time zone/time sync between client and server, and accounting for latency.
//
// The latency adjustment is pessimistic: if we get 12:00:25 back with a latency
// of 5 seconds, we know the current server time is between 12:00:25 (5 sec req,
// 0 sec res) and 12:00:30 (0 sec req, 5 sec res). We assume it is 12:00:30 so
// that trusting the server time may make you early, but never late.
//
// This is a singleton class: get it with [AKServerTime sharedServerTime].

#import <Foundation/Foundation.h>


@interface AKServerTime : NSObject {
  NSTimeInterval _offset;
  NSTimeInterval _latency;
}

+ (id)sharedServerTime;

- (void)syncWithClientDate:(NSDate *)clientDate serverDate:(NSDate *)serverDate andLatency:(NSTimeInterval)latency;
- (NSDate *)date;

@end

AKServerTime.m

#import "AKServerTime.h"

static AKServerTime *sharedServerTime = nil;

@implementation AKServerTime


#pragma mark -
#pragma mark Date

- (void)syncWithClientDate:(NSDate *)clientDate serverDate:(NSDate *)serverDate andLatency:(NSTimeInterval)latency {
  NSDate *pessimisticServerDate = [serverDate addTimeInterval:latency];
  NSTimeInterval offset = [pessimisticServerDate timeIntervalSinceDate:clientDate];
  
  // If we've synced with smaller latency before, and the offset is not
  // large enough (under 5 seconds) to suspect something like a DST change,
  // keep the old value as more reliable.
  if (_latency && _latency < latency && (fabs(offset-latency) < 5.0)) {
    NSLog(@"SERVER TIME: higher latency at %f, remaining offset small at %f - keep old value", latency, fabs(offset-latency));
  } else {  
    _latency = latency;
    _offset = offset;
  }
  NSLog(@"SERVER TIME: local now %@ server now %@ (latency %f, offset %f)",
        [NSDate date], [self date], _latency, _offset);
}

- (NSDate *)date {
  return [[NSDate date] addTimeInterval:_offset];
}


#pragma mark -
#pragma mark Singleton
// http://iphone.galloway.me.uk/iphone-sdktutorials/singleton-classes/

+ (id)sharedServerTime {
  @synchronized(self) {
    if(sharedServerTime == nil)
      [[self alloc] init];
  }
  return sharedServerTime;
}

+ (id)allocWithZone:(NSZone *)zone {
  @synchronized(self) {
    if(sharedServerTime == nil)  {
      sharedServerTime = [super allocWithZone:zone];
      return sharedServerTime;
    }
  }
  return nil;
}

- (id)copyWithZone:(NSZone *)zone {
  return self;
}

- (id)retain {
  return self;
}

- (unsigned)retainCount {
  return UINT_MAX; //denotes an object that cannot be released
}

- (void)release {
  // never release
}

- (id)autorelease {
  return self;
}


@end