Introduction

ServerDate is used in web pages to make the server's clock available to the client's browser via Javascript.

You can use it when you want to display the current time but don't trust the browser's clock to be accurate, or to synchronize events for different users of your web site by synching them all to the server's clock.

Usage is simple. Add one <SCRIPT> element to your web page and then you can use the ServerDate object in Javascript just like you use the built-in Date object, except that it reflects the server's time instead of the client's.

Requirements

ServerDate has been tested in the following browsers:

• Chrome 23.0.1271.64
• Firefox 16.0.2
• Safari 6.0.1 (7536.26.14)
• Internet Explorer 11.0.9600.18015

Installation

Include the following <SCRIPT> element in your web page:

<SCRIPT>
  window.ServerDate = {
    // Show debug information in console
    debug: false,
    // Max synchronizing iterations. 10 it's a good number of iterations.
    iterations: 10,
    // After a synchronization there may be a significant difference between our
    // clock and the server's clock.  Rather than make the change abruptly, we
    // change our clock by adjusting it once per second by the amortizationRate.
    amortizationRate: 25, // ms
    // The exception to the above is if the difference between the clock and
    // server's clock is too great (threshold set below).  If that's the case then
    // we skip amortization and set the clock to match the server's clock
    // immediately.
    amortizationThreshold: 2000, // ms
    // After the initial synchronization the two clocks may drift so we
    // automatically synchronize again every synchronizationIntervalDelay.
    synchronizationIntervalDelay: 60 * 60 * 1000 // ms
  };
</SCRIPT>
<SCRIPT src="ServerDate.js"></SCRIPT>

Usage

You may then use ServerDate as you would use the Date object or one of its instances, e.g.:

> ServerDate()
"Mon Aug 13 2012 20:26:34 GMT-0300 (ART)"

> ServerDate.now()
1344900478753

> ServerDate.getMilliseconds()
22

There is also a new method to get the precision of ServerDate's estimate of the server's clock (in milliseconds):

> ServerDate.toLocaleString() + " ± " + ServerDate.getPrecision() + " ms"
"Tue Aug 14 01:01:49 2012 ± 108 ms"

You can see the difference between the server's clock and the browsers clock, in milliseconds:

> ServerDate - new Date()
39

There is no constructor because it doesn't make sense to create more than one instance of ServerDate.

Methods from Date to change the time, such as setMinutes, are not defined:

> ServerDate.setMinutes
undefined

ServerDate is synchronized with the server's clock when it is first loaded and then re-synchronizes itself from time to time to keep the two clocks from drifting apart.

References

• "Probabilistic clock synchronization" by Flaviu Cristian Cristian, F. (1989), "Probabilistic clock synchronization", Distributed Computing (Springer) 3 (3): 146–158, DOI:10.1007/BF01784024
• MikeWyatt's answer and Willem Mulder's comment in Sync JS time between multiple devices
• Rob W's answer to How to synchronise a client webpage timer with the server

Copyright

Copyright 2012 David Braun

This file is part of ServerDate.

ServerDate is free software: you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

ServerDate is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License along with ServerDate. If not, see http://www.gnu.org/licenses/.