You add NTP servers under Administration > System Settings: Date and Time. These configuration options are available:

Displays the controls to add a server.

Specifies the hostname or IP address for the NTP server. You can connect to an NTP public server pool: for example, 0.riverbed.pool.ntp.org. When you add an NTP server pool, the server is selected from a pool of time servers. The IP address can be either IPv4 or IPv6. For IPv6 specify an IP address using this format: eight 16-bit hexadecimal strings separated by colons, 128-bits. For example: 2001:38dc:0052:0000:0000:e9a4:00c5:6282

You don’t need to include leading zeros. For example: 2001:38dc:52:0:0:e9a4:c5:6282

You can replace consecutive zero strings with double colons (::). For example: 2001:38dc:52::e9a4:c5:6282

Specifies the NTP server version from the drop-down list: 3 or 4.

Connects to the NTP server or disconnects from the NTP server by selecting either Enabled or Disabled from the drop-down list.

Specifies the MD5 or SH1 key identifier to use to authenticate the NTP server. The valid range is from 1 to 65534. The key ID must appear on the trusted keys list.

Specifies the authentication key type: MD5 or SHA1.

Adds the NTP server to the server list.