Please note that the mentioned functionalities are inbuilt and you could use Delete or BackSpace buttons to delete the needed content without adding the client code you are using. Could you please elaborate on what exactly you need to achieve with the client script you are implementing so we could provide proper solution for all browsers.
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I go into a web page with a few RadDateInput controls
I enter a date into one of them
I then try to erase the box with either the delete key or the backspace
IE, Safari and Chrome let me - in Firefox and Opera the textbox is locked