There is some good information on setting the switch System Time from an SNTP Server starting on page 115 of the User Guide.

User Guide:

support.dell.com/.../en_ug.pdf

SNTP Server Types

The switch can accept time information from the following server types:

• Unicast

Polling for Unicast information is used for polling a server whose IP

address is known. This is the preferred method for synchronizing device

time, as it is most secure.

Up to eight SNTP servers can be defined.

If this method is selected, SNTP information is accepted only from SNTP

servers defined in the SNTP Servers page.

Time levels T1 - T4 (see the "Algorithm for Selecting Designated SNTP

Server" on page 118 section) are used to determine from which server time

information is accepted.

If Unicast polling is not enabled or if no servers are defined on the device,

the device accepts time information from any SNTP server of the type that

is enabled, which responds.

• Anycast

Polling for Anycast information is used when the SNTP server’s IP address

is not defined or it cannot be reached. If this method is enabled, time

information can be received from any SNTP server on the network. The device time and date are synchronized when it proactively requests

synchronization information.

Anycast polling to get time information is preferable to Broadcast polling,

because it is more secure.

Time levels T3 and T4 are used to determine from which server time

information is accepted.

• Broadcast

Broadcast information is used if receiving Broadcast packets has been

enabled, and one of the following situations occurs:

– The SNTP server IP address has not been defined.

– Several time-information packets are received and the Broadcast time

is best according to the algorithm defined in "Algorithm for Selecting

Designated SNTP Server" on page 118.

Broadcast is the least secure method of receiving time, because it is both

unsecured and the time information was not specifically requested by the

device. Anycast is also unsecured, but time-information packets are only

accepted if they were requested.