SNTP implementation for iOS
Objective-C Ruby


A network time protocol client (and an application testbed for iOS). This is a continues to be a work in progress.

Created by Gavin Eadie on Oct 17, 2010


December 20, 2016: (version 1.1.4) improvements have been made in a few areas:

  • the use of pool ntp server host names is strongly discouraged so they have been removed from this code and documentation. Read the NTP Pool Project page at for context. NOTE: The library will query NO servers in its new default state .. now a ntp.hosts file MUST be provided.

February 1, 2016: (version 1.1.3) improvements have been made in a few areas:

  • arithmetic operating on an NTP 64-bit time has been improved slightly.
  • the delegate callback from NetAssocation now runs on the main thread, which allows it to modify any UI component (illegal from a background thread).
  • a "receive packet filter" has been added, but not yet invoked. This will used to drop UPD packets that don't pass validation.
  • [NetAssociation finish] now invalidates its timer so that association plays no further part in time derivation.
  • upgrade sources to most recent Objective-C conventions.

Getting a Quick Timecheck

ios-ntp is often (mostly?) used to make sure someone hasn't fiddled with the system clock. The complications involved in using multiple servers and averaging time offsets is overkill for this purpose. The following skeleton code is all that is needed to check the time. If you want some more assurance of accuracy, repeat the operation a few time; if you want to continue to watch the clock, you might invoke this every five minutes:

    #import "ios-ntp.h"

    @interface ntpViewController : UIViewController <NetAssociationDelegate>


    @implementation ntpViewController
    - (void)viewDidLoad {
        [super viewDidLoad];

        netAssociation = [[NetAssociation alloc]
        initWithServerName:[NetAssociation ipAddrFromName:@""]];
        netAssociation.delegate = self;
        [netAssociation sendTimeQuery];

    - (void) reportFromDelegate {
        printf("time offset: %5.3f mSec", netAssociation.offset * 1000.0];

January 17, 2016: (version 1.1.2) minor cleanup for Xcode 7.x

July 22, 2015: (version 1.1.1) ios-ntp has contained a resource file (called ntp.hosts) which contained a list of time server hosts to be used for querying the time. That file has been removed in this release.

The logic is that, since ios-ntp can now be added to a project via CocoaPods, any local changes made to that file will be overwritten the next time the ios-ntp pod is updated and, since ios-ntp already contains a built-in list of time servers, removing this file from the pod should not impact the behavior of the ios-ntp code.

If you want to use your own list of time servers, you need to create a file containing time host names, one per line, name it ntp.hosts and place it in the main bundle of your application (the sample app ios-ntp-app does this to use the server at

June 10, 2015: (version 1.1) I recently discovered a re-entrancy bug when John Grismore brought my attention to inaccuracies in reported network time offsets. When a NetAssociation notified the NetClock that it had a new time offset, that event might interrupt the offset averaging that NetClock does causing the averaging to break.

In fact, this mechanism isn't optimal anyway! The notifications that cause offset averaging arrive at NetClock constantly whether the result is used or not. We're keeping the NetClock network time offset property up to date whether we need it or not. Better would be to perform the averaging only when the offset NetClock property is called for, and that is how ios-ntp now works.

The API is not changed, so your application should require no changes.

February 22, 2015: Several important changes have been made including one that will be helpful for those who want to get a quick one-time value of the difference between system time and network time.

Before this change, ios-ntp would use time estimates from a set of NetAssociations (one per time servers), constantly determining the best time by sampling these values. This is the model for computers which have a continuous low level task monitoring the time. Application in iOS often have a different need; they are more likely to want an fast estimate of the time on demand. To provide the ability to the developer, access has been provided to use a NetAssociations directly

An NetAssociation can now be asked for one measure of the time from one time server so an iOS app can create an NetAssociation, use it to get the time, and be done.

This code operates on 32-bit and 64-bit iOS devices.

This code requires iOS 7, or higher.


The clock on the oldest iPhone, iTouch or iPad is not closely synchronized to the correct time. In the case of a device which is obtaining its time from the telephone system, there is a setting to enable synchronizing to the phone company time, but that time has been known to be over a minute different from the correct time.

In addition, users may change their device time and severely affect applications that rely on correct times to enforce functionality, or may set their devices clock into the past in an attempt to dodge an expiry date.

This project contains code to provide time obtained from standard time servers using the simple network time protocol (SNTP: RFC 5905). The implementation is not a rigorous as described in that document since the goal was to improve time accuracy to tens of milliSeconds, not to microseconds.

Computers using the NTP protocol usually employ it in a continuous low level task to keep track of the time on a continuous basis. A background application uses occasional time estimates from a set of time servers to determine the best time by sampling these values over time. iOS applications are different, being more likely to want a one-time, quick estimate of the time.

ios-ntp provides both the continuous and on-demand modes of operation. The continuous mode uses multiple 'associations' with time servers which use timers to repeatedly obtain time estimates. These associations can, however, be used by the developer to get one time from one server.


The code can be incorporated as source code or as a framework in an Xcode project. The framework usage is temporarily unavailable but will be restored soon.

More to come about using a framework.

Download the ios-ntp project, add the necessary to your project, build and run. You will need:

    #import "ios-ntp.h"

where ios-ntp is referenced.

Continuous Mode

Simply create a NetworkClock. As soon as you create it, the NTP process will begin polling the time servers in the "ntp.hosts" file. You may wish to start it when the application starts, so that the time is well synchronized by the time you actually want to use it, just call it in your AppDelegate's didFinishLaunching method.:

    NetworkClock * nc = [NetworkClock sharedNetworkClock];

then wait at least ten seconds for some time servers to respond before calling:

    NSDate * nt = nc.networkTime;

It will take about one minute before untrustworthy servers start to get dropped from the pool.

It would probably be better if NetworkClock called back to a delegate method, like NetAssociation does below, when it had a good time but that's not how it works, yet, so you have to wait till things settle down.

On Demand Mode

This usage is slightly more complicated. The developer must create an NetAssociation (with some specified time server), and then tell it get the time from that server. The association uses a delegate method to return itself with time information.

    netAssociation = [[NetAssociation alloc] initWithServerName:@""];
    netAssociation.delegate = self;
    [netAssociation sendTimeQuery];


    - (void) reportFromDelegate {
       double timeOffset = netAssociation.offset;


iOS is an event driven system with an emphasis on rapid response to gestures at the cost of other activity. This encourages the extensive use of design patterns like notification and delegation so, I think, the calculation small time differences in this environments suffers as a result.

Empirical observations of one time server shows some an occasional time offset that is significantly greater than its usual values; the calculated standard deviations of any one server's offsets is higher than would be expected, and I don't know the cause of this.


The MIT License Copyright (c) 2010-2015, Ramsay Consulting


More to come about building a framework.


November 19, 2014: A large update was made today to bring ios-ntp into the modern world. The changes do include one bug fix, but are mostly related to making the code comply with the recent Xcode changes and requirements.

Some of jbenet's "Usage" notes below aren't completely accurate as a result of these changes, and I will update the text soon.

Finally, note that this code was first written when there were only 32-bit iOS devices. As I write this there are still 32-bit devices which run the latest version of iOS (iPhone 4S, for example), but all newer iOS devices have a 64-bit architecture (iPhone 6, for example), and Apple requires that this be supported.