rdate is a Linux command and the network time protocol for immediate date and time setup from another machine. The clocks are synchronized by entering rdate with the -s switch (command without switch just checks the time but does not set anything).
rdate is the older standard that in many cases is now replaced by ntp/ntpd. Unlike ntp, rdate sets the new time instantly and is more suitable in situations like initial setup. Abrupt change of the clock settings may cause problems for some software (ntp changes time gradually). However ntp may not be available in some systems and during some initial setups is actually desired to set the correct time as soon as possible.
Under Linux, rdate does not have its own network service and is supported by the standard time service instead. It uses port 37 and can work via TCP or UDP (with -u switch).
This is tutorial
How to public RDate Server.
Public RDate server need two packages: xinet & ntp