I would suggest using the value returned by
DATE rather than
CDATE.
DATE stores the current date & time as a Julian date value, with the value following the decimal point representing the fraction of a day. Hence, to add 30 minutes you simply need to divide 30 by the number of minutes in a day (1440) and add this to the value returned by the
DATE system variable.
The result can then be represented in a human readable format using the DIESEL
edtime function, evaluated from a menu command expression, e.g.:
(menucmd "m=$(edtime,$(+,$(getvar,date),$(/,30.0,1440.0)),yyyy-mo-dd hh:mm:ss)")