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SNTP implementation for iOS
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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


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 repeated time estimates from a set of 'associations' with 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. The need for time that iOS apps have is different; they are more likely to want an fast estimate of the time on demand. To provide ability to the developer, access has been provided to use the 'associations.'

An association can now get one measure of the time from one time server so an iOS app can create an association, 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 (if the file isn't found, a tasteful set of default servers will be used). 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.

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