You cannot change system time in a VPS because all VPSes on the hardware node have the same system time. However, it is possible to change the default system timezone inside a VPS, for example, by replacing /etc/localtime with the file from /usr/share/zoneinfo which contains a description of your timezone.
For example if you want to change the timezone to NZ, you can do this via SSH:
cp NZ /etc/localtime
The server will ask you:
cp: overwrite `/etc/localtime’?
Answer with y and your timezone will be changed.