Using System Time in Shell Script

This shell script shows to print any one of the following message in infobox using dialog utility, if installed in your system, If dialog utility is not installed then use echo statement to print message :

Good Morning
Good Afternoon
Good Evening , according to system time.

temph=`date | cut -c12-13`
dat=`date +"%A %d in %B of %Y (%r)"`

if [ $temph -lt 12 ]
then
    mess="Good Morning $LOGNAME, Have nice day!"
fi

if [ $temph -gt 12 -a $temph -le 16 ]
then
    mess="Good Afternoon $LOGNAME"
fi

if [ $temph -gt 16 -a $temph -le 18 ]
then
    mess="Good Evening $LOGNAME"
fi

if which dialog > /dev/null
then
    dialog --backtitle "Linux Shell Script Tutorial"\
    --title "(-: Welcome to Linux :-)"\
    --infobox "\n$mess\nThis is $dat" 6 60
    echo -n "                            Press a key to continue. . .                        "
    read
    clear
else
    echo -e "$mess\nThis is $dat"
fi