When it comes to synchronizing a computer network there are several choice to ensure each device is running the same time. NTP (Network Time Protocol) is the preferred choice of time synchronization protocols but there are a multitude of methods in how NTP receives the time.
The NTP Daemon is installed on most operating systems such as windows and applications such as Windows Time are quite capable of receiving a source of UTC time (Coordinated Universal Time) from across the internet.
UTC time is the preferred time source used by computer networks as it is kept true by atomic clocks. UTC, as the name suggests, is also universal and is used by computer networks all over the world as a source to synchronize too.
However, internet sources of UTC are to recommended for any organisation where security and accuracy are a concern. Not only can the distant from host (internet time server) to the client (your computer network) can never be accurately measured leading to a drop in precision. Furthermore, any source of internet time will need access through the firewall (usually through the UDP 123 port). And by leaving this port open, malicious users and hackers can take advantage and gain access to the system.
Dedicated NTP time servers are a better solution as they receive the time from an external source. There are really two types of NTP server, the radio reference time server and the GPS time server.
Radio reference time servers use signals broadcast by places like NPL (National Physical Laboratory in the UK) or NIST (National Institute of Standards and Time). While these signals are extremely accurate, precise and secure they are affected by regular maintenance on the transmitters that broadcast the signal. Also being long wave they are vulnerable to local interference.
GPS time servers on the other hand receive the time directly from GPS satellites. This GPS time is easily converted to UTC by NTP (GPS time is UTC – 17 seconds exactly as no leap seconds have been added.) As the GPS signal is available everywhere on the earth 24 hours a day, 365 days a week, there is never a risk of a loss of signal.
A single dedicated GPS time server can synchronize a computer network of hundreds, and even thousands of machines to within a few of milliseconds of UTC time.