By default the time is controlled by Proxmox host. So if you want to install a NTP server or modify the time on a OpenVZ CT you must enable time capability.

1- Login in Proxmox console and stop the target OpenVZ CT:

# vzctl stop CTID

NOTE: CTID is the CT's ID. To list availables CTs: vzctl list

2- Enable time capability:

# vzctl set CTID --capability sys_time: on --save

3- Start the CT:

# vctl start CTID