Configuring the time zone on IP phones
3CX PhoneSystem includes functionality to easily set the Time Zone and DST (Daylight Savings Time) parameters on phones via provisioning. 3CX PhoneSystem is delivered preconfigured with default values for US Eastern Time. You can manage Time Zone settings from the 3CX Management Console in the “Settings->Phone Provisioning” page.
Time Server
You can change this value to some other public FQDN or IP Address, if the phone has access to the internet for the NTP protocol.
Alternatively, you can set this to some internal FQDN or IP Address. Please refer to this article for further details: http://www.3cx.com/blog/docs/windows-sntp-ntp-server/
Time Zone for the Different Models
For each of the different provision-able phone brands, you can select the appropriate Time Zone value from the drop-downs. The example above is for US Mountain Time.
Daylight Savings Time
If your Time Zone requires Daylight Savings Time to be configured, check the “Enable Daylight Savings Time” option, and specify the dates when DST starts and ends. The “Offset” field default value of 60 minutes is valid for most time zones. The example above shows the DST settings for US Mountain Time in 2011.
[...] This article contains tables listing the fields necessary for each brand, and also the relative valid values for that brand. [...]