Voip phones get the time from Network Time Servers. Network time servers are in the internet and provide the time in Coordinated Universal Time or UTC. The phone is set with an offset for the geographic location of the phone to adjust the date and time to be correct.

When the date and time display incorrectly, it's usually because when the phone went to the designated time server, in OnSIP's configurations it's pool.ntp.org, it either did not receive an updated time date (often because the server is overloaded, or the request never left the network). The phone will try again, but by default it waits 8 hours for the next check. Normally it will update on the next attempt, but if it does not, then the cause is usually either a DNS issue where the request is not leaving the network successfully or if there is a NTP server on your local network, may be having a conflict.

It's best to be patient when the time is incorrect, because every time the phone is rebooted, if it fails after the reboot, the timer starts new and it's a longer wait for the next check.

It's also good to know that because the time and date on the phone are incorrect does not affect the operation of calling in any way. The time and date are there mostly as a comfort feature.