For your comfort, this page is excellent to start with.We've provided a list of the most common causes of NTP time sync troubles.This includes its ability to schedule tasks and notify you of calendar events, as well as time-stamp files and events, and properly communicate with online services using some forms of encryption.Therefore, if the clock on your Mac is off, then you might not only be late for a meeting, but might also not be able to send or receive messages and other Web content.
First, ensure you have an internet connection, and then ensure you can communicate with an official and dedicated time server.
Check which one applies yours and follow the proposed steps to make time service successful. One reason for NTP synchronisation problems may be a firewall or port filter that is blocking the ports the programs use to communicate (by default UDP port 123).
For example in Windows 8, check firewall settings in Control Panel -Advanced settings.
With your time server route confirmed, go to the Date & Time system preferences, and then enter the URL of the time server in the drop-down menu within the Date & Time tab (Apple provides pre-defined servers for its US, Asian, and European time servers in this menu).
You have landed here most probably while searching for solution to your NTP synchronization problem, right?