How to validate a Gridview column based on another checkbox column

Hi,
I have a gridview which displays following columns:
TaskID, TaskDescription, IsComplete, DateOfCompletion
Now what I want to achieve is, when user clicks IsComplete checkbox, she must enter the date of completion. So, I want a validation (client side) on DateOfCompletion if the checkbox is selected (checked). And, I also want to use Validator Callout Extender if possible.

How can I achieve this?
Thanks in advance,
Ashish Tilak

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