Almost every site I go to has the same problem, the time on the phone display does not match the time on the PC screen. What are the reasons for that issue? Well first of all you have to understand how the time system in your PBX and your Windows environment are working.
The phone system (PBX) usually gets its time signal from an external source and is not configured manually. In 90% of the cases I configure my customers to use the ISDN trunk as time source. The time the provider provides on that line is 100% accurate and you receive it without delay. The time is updated every time a user establishes a phone call.
The Windows Domain has its own time server. By default this is the Domain Controller or Operations Master. This servers provide time to the other clients in the network but are by default not configured to update their information from an external time source and the admin has to set the time manually or the time will just go wrong after a couple of month operation but no one cares or knows what to do.
The following batch script is what I use to solve this problem and setup a link to an external time source that is updated every hour. On your DC just create a new text file rename it to time.bat and open it with an editor of your choice. Enter the following commands:
w32tm /config /syncfromflags:manual /manualpeerlist:time.windows.com
w32tm /config /update
They will set the time source to the Microsoft NTP server (time.windows.com) which is default on a workstation and will resynchronise the time with this server. Now all you need to do further is to setup a scheduled task to repeat this every hour or at least daily.
As alternative you can set your PBX to sync with the DC as well just change the NTP setting on it, but I cannot recommend a solution with wrong time information. Look @ Microsoft for further information on that.