I figured out this by myself.
Add the following function into your PBS batch job script to get the job submit time.
getsubmitdate(){
local datestring=`qstat -f $PBS_JOBID | grep -F qtime | awk '{for(i=3;i<8;i++) printf $i" "}'`;
local result=`date -d "$datestring" +%Y%m%d` ;
local outputvar=$1 ;
if [[ "$outputvar" ]] ; then
eval $outputvar="'$result'"
else
echo "$result"
fi
}
getsubmitdate SUBMITDATE
echo $SUBMITDATE