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