mysql querry of timediif to calculate difference in 3rd col

In mysql I have two column of time tcal and Tpost I just need there difference in third column Rtime. I tried various queries but nothing work. I am very confused. I used these commands but nothing is woking. First Try $tresult = mysql_query("SELECT TIMEDIFF(tcal,Tpost) FROM $usertable"); mysql_query("UPDATE $usertable SET Rtime = $tresult"); [/code] Second Try mysql_query("UPDATE $usertable SET Rtime = TIMEDIFF(tcal,Tpost"); Only find that this code is working: mysql_query("SELECT TIMEDIFF(tcal,Tpost) FROM $usertable"); But this code can only use with echo but i need to put the result back into my database I need to use this time difference result in third coloumn.. If you have any other idea to calculate difference then discuss with me. I can share my team viewer id.. I am just stuck in this function its very simple usually takes 10-20 min to complete that's why my budget is very low..