You may find the phone’s admin guide a little confusing when it comes to Self-Defined Time Zone setting under Date and Time Setting. This article shows you the steps to complete
a. Navigate to the phone‘s web interface > Setting > Preferences > Date and Time
b. Time zone: select Self-Defined Time Zone
c. Go to this link to find out your country standard time and UTC time.
For eg. if you resides in Dhaka, Bangladesh, the time zone is BST+6 with no daylight saving time. However, at the Self-Defined Time Zone field, key in BST-6 and click Save and Apply.
d. Navigate to the Status page > System Info > verify System Time upon changes.