National Physical Laboratory

Synchronise your computer with NPL's atomic clock

Time synchronisation of computers to UTC(NPL)
Image courtesy of iStockphoto

How does a computer know the time and date? How does it know when the clocks are next due to go forward or back? Or when there will next be 61 seconds in the minute?

A computer time-stamps every event, such as an e-mail being sent or a file saved, by taking the time from its internal clock. This clock is usually an inexpensive quartz oscillator that can drift by several seconds over a day and gradually build up an error of many minutes. If the computer is to tell the time accurately it must set its internal clock regularly against a trusted external source of time, which must be both accurate and reliable. NPL offers two services that provide access to just such a source - the UK national time scale located at NPL, which is based on a group of atomic clocks.

  • The Telephone Time Service (TTS) allows a computer to set its internal clock by direct telephone connection to a server at NPL. A call to the NPL TTS can be made at any time of the day or night, and it allows a computer equipped with a suitable modem and software to set its clock to UK local time and to learn in advance about important time changes. It is particularly valuable in situations where the computer is not connected to the Internet, for example in a security system where the equipment needs to be tamper-proof, or where a reliable source of UK local time is required. A User's Guide to the NPL Telephone Time Service is available that explains how to obtain and install software that will allow a computer to use the service, and includes a full description of the time code for anyone wishing to write their own software to access the service. It can be downloaded here (free of charge) PDF File
  • The Internet Time Service (ITS) allows a computer to set its internal clock by connecting over the Internet to a server at NPL that transmits a time code using the well-established Network Time Protocol (NTP). Either a dial-up or broadband Internet connection can be used. A User's Guide to the NPL Internet Time Service provides more information and explains how to obtain and install software that will allow a computer to use the service. It can be downloaded here (free of charge) PDF File

The two services are complementary; the differences between them are summarised in the table below.

Telephone Time Service Internet Time Service
Means of access Modem connected to external telephone line Internet service provider
Phone number/
Internet addresses
020 8943 6333
Availability Worldwide Worldwide
Cost Standard call charge Free
Time reference UK civil time (UTC or BST) UTC
Accuracy Around 30 ms Around 20 ms
Software NPL Phoneclock for Windows PCs Widely available
User Guide TTS User Guide PDF File ITS User Guide PDF File

Changes to the Telephone Time Service

Since 16 August 2015, the previous method of accessing NPL TTS using a UK premium-rate telephone number, 0906 851 6333, has been replaced by a direct-dial number, 020 8943 6333. Calls to the service are now charged at the standard network rate, and there are no additional costs for using the service.

Software for the Telephone Time Service

A program called 'NPL Phoneclock' has been produced by NPL for use with the NPL Telephone Time Service. NPL Phoneclock is intended for use on PCs running most current versions of the Microsoft Windows operating system. It is compatible with 32-bit versions of Microsoft Windows, but will not run on 64-bit systems.

NPL Phoneclock can be downloaded here (free of charge)


Customer Services tel: +44 20 8943 6796

Last Updated: 18 Nov 2015
Created: 13 Jun 2007


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