Finding a source of time to synchronise a computer network to can be a challenge as there are a myriad of online time sources, all pertaining to be accurate and reliable; however, the truth can be rather different with many online sources either in too much demand, too far away or inaccurate.
NTP (Network Time Protocol) requires a source of UTC time (Coordinated Universal Time) which is kept true by atomic clocks. Online time sources are not themselves atomic clocks but NTP server devices that receive the time from an atomic clock which is then relayed to the devices that connect to the online time server.
There are two types of online time server: stratum 1 devices devices that receive the time directly from an atomic clock, either using GPS or a radio reference signal. Stratum 2 devices on the other hand are one step further away in that they are receive their time from a stratum 1 time server.
Because of demand, finding an online stratum 1 time server is next to impossible, and those that do take request usually do so under a subscription, which leaves the only choice for most people being a stratum 2 device.
There are plenty of resources on the internet that provide locations for online time servers such as http://support.microsoft.com/kb/262680
But there are drawbacks to using such devices; firstly, online stratum 2 time sources can't be guaranteed and several surveys taken have found that the reliability and accuracy of many of them can't be taken for granted. Secondly, online sources of time require an open firewall port which can be manipulated by malicious bots or users leading to security risks.
A far better solution for most networks is to install your own stratum 1 NTP server. These time server devices sync to atomic clocks outside the firewall (using GPS or radio signals) and therefore are not security risks. They are also accurate to a few milliseconds ensuring the network will always be accurate to UTC.