Thank you. This works also but it does not begin the next group by one so this seems to work after the first group is created.
if {Command.timediff} in [0 to 30] then "0-"+Totext(Ceiling({Command.timediff},30),0,'')
else
totext(ceiling({Command.timediff},30)-29,0,'')+'-'+Totext(Ceiling({Command.timediff},30),0,'')