Help with dropdown menu

[JaneDoe]

I got a quick question for everyone. I am trying to build an employee scheduler and wondering how would I have to code the following feature: if i pick a time from dropdown menu1, lets say 9:00am, and then pick another time from dropdown menu2, lets say 5:00pm, i would like the program to automatically calculate the total time, which would be 8, and put it into a textfield without having to click a submit button. So, in other words, as soon as a value for dropdown menu2 is chosen, the total time would automatically be calculated into the textfield. Any ideas anyone?

[Hiccup]

You would have to use javascript for this to work. Do a google search on "javascript, forms, addition, subtraction" and you'll find out how.

Question: Why bother?

Let php calculate it once the entire form is submitted. I ran into this before in one of my scripts and realized that it's just more effective, simple and efficient for the user to input the start and end and let php handle the calculations.

Just my 2 cents.

[JaneDoe]

... thanks for the reply. To answer your question, i want it automatically calculated because it would eliminate the user from pressing a calculate button repeatedly.