There are many public time servers available over the Internet.  Access to these public time servers is free of charge.  While public time servers are certainly less costly - accurate, reliable and secure time is best provided by a dedicated time server that resides under your control inside your network security perimeter.  Using public time servers available over the Internet is not recommended for the following reasons:

1.  Setting up your firewall to accept NTP packets (which is based on UDP/IP) introduces a security risk that many Network Administrators are not willing to take.

2.  Time accuracy degrades because of indeterminate network latency, up to 100 milliseconds is typical.