For plesk version 10 and above
=========================
1. Log into plesk control panel.
2. Select "Tools & Settings" option.
3. Select "System Time" option from "General Settings" section.
4. Enter the value for "Hours","Minutes" and "Seconds" fields.
5. Click "OK" button.
For plesk version 9
=============
1. Log into plesk control panel.
2. Select "Settings" from "Main menu".
3. Select "System Time" from "General" section.
4. Enter the value for "Hours","Minutes" and "Seconds" fields.
5. Click "OK" button.