getting sum in textbox based on values entered in another textbox in a gridview

i have 2 template field columns time and total_time in a gridview..

time column has a label in the item template and a textbox in th edit item template..

similarly, total_time has a label in the item template and a textbox in the edit item templte..

what i need is, when the user enters a value in the time textbox,it must get updated in the total_time textbox.. and on edit,when the user changes the value of the time textbox ,dis value must be ADDED to the total_time textbox.. both should be displayed in the label also..

plzzz help asap!!