On changing one field, update another field in the same form

I have this checkbox named "Completed" and a field CompletedDate. When the user checks the checkbox, I want to update CompletedDate to the current date. How do I do that?

I tried with a screen action that is activated when the checkbox is changed, but to no avail.
You can use the onChange of the checkBox attached to an action. Be sure that you're filling the form value and not something else. Remember to Ajax refresh the place where you have the expression, or input, of the CompletedDate.

Best regards
Thanks. I used the onChange to fire the screen action that sets the CompletedDate. I guess the issue is the "Ajax refresh": I do not know how to do that.

The ajax Refresh should be done on the input (or form) that you want to Refresh. thats a simple example x)
Okey! I found out how to do the Ajax refresh. Thanks for your help.