custom form control custom client side validation in Kentico

We've created a custom FormEngineUserControl to capture date input using 3 text inputs for day/month/year.

On the server we override the bool IsValid() method which works fine and displays the error message if invalid.

However we want to use client side validation also, we can use a CustomValidator control and assign a ClientValidationFunction to call a JS method however this is not then combined with the server validation function and we end up with two validation messages, one that removes when client validation passes, and one that only disappears when the form is re-submit, basically rendering the client validation useless.

Is there no way to register a client validation method with a custom form control? that will then be combined with the server method and error label etc?

Also I posted this to stack overflow tagged against Kentico thinking it would be integrated here but it seems they are two different systems? If that's not the case then I apologise for the double post.