keydown? dynamic change?

I have a form that is populated with a previous month's data, some textboxes, and a blank area.

The user is supposed to put new data in the textbox and a subtraction should be done on that and the previous month's data and the new data should appear in the blank area (table data area).

Question is, how do I accomplish this? I'm converting this from Access and it can update this as the user types or once the user presses enter. Is there a way to do this without refreshing the page?

Not in PHP, only in JavaScript.

Thanks. Someone else also told me that. I didn't realize PHP was a server-side only scripting language.