Shell Programming and Scripting

I am learning Unix for the first time. I am playing around with my default profile file to add some script in the KSH. The time format is GMT and I am in eastern time zone. I am trying to script some greetings based upon the time of day using the IF/ELIF conditions. I am getting errors. I have tried researching to see what I am doing wrong but am getting nowhere. I am sure there is someone out there that can show me the way. Here is the script I am trying to run:

if [ \( `date +%R` < 16:00 \) -a \( `date +%R` > 2:00 \) ]
echo "Good Morning"
elif [ \( `date +%R` > 15:59 \) -a \( `date +%R` < 22:00 \) ]
echo "Good Afternoon"
else
echo "Good Evening"
fi

I appreciate any help anyone can give me here to get this to work.
Thanks....

something to start with:

Code:

#!/bin/ksh

hour=$(date +%H)
echo "hour->[${hour}]"

case ${hour} in
 @(0[0-9]|1[1-2]) )  echo "Good Morning" ;;
 @(1[3-7]) )         echo "Good Afternoon" ;;
              *)     echo "Good Evening" ;;
esac

Code:

TZ='EST5EDT4'

will then change from GMT to EST(EDT). Which I think may be another problem. What system are you on? because the TZ variable set this way works but may not correctly set daylight time - eg. Linux has this issue.