ASP.NET MaskedEditExtender needs to show mm/dd/yyyy when user clicks on textbox

I am using the MaskedEditExtender control for a textbox field that contains Date of Birth. I want the mask to show "mm/dd/yyyy" when the user clicks on the textbox when it is empty. Right now it only shows "__/_/__"

I believe you can set the PromptCharacter to something else, but I don't think you can specify multiple prompt characters in one textbox. Because of this I would say your options are limited to:

• Javascript schenanigans where you replace the "_" with m,d or y depending on where it is in the text box.
• Multiple textboxes