You have to evaluate the var before adding to it.

//set %var $+ $date $time
- %var23/04/2003 16:56:46

Now to add to it,
//set %var $+ $date $eval($+(%,var,date),2) $time
or
//set $+(%,var,$date) $eval($+(%,var,date),2) $time
- %var23/04/2003 16:56:46 16:56:51


raw 386:*:{ set %q $+ $2 $eval($+(%,q,$2),2) $3 }