How to decode viewstate

eval() is not necessary. This will work fine:

var date = new Date(parseInt(jsonDate.substr(6)));

The substr() function takes out the /Date( part, and the parseInt() function gets the integer and ignores the )/ at the end. The resulting number is passed into the Date constructor.

I have intentionally left out the radix (the 2nd argument to parseInt); see my comment below.

Also, I completely agree with Rory's comment: ISO-8601 dates are preferred over this old format - so this format generally shouldn't be used for new development.

For ISO-8601 formatted JSON dates, just pass the string into the Date constructor:

var date = new Date(jsonDate); //no ugly parsing needed; full timezone support

My 2 cents: Instead of using the simplicity of ViewState sandbag use the simplicity of Session - which really is the same simplicity and you won't have to bother worrying about page refresh and stuff. Moreover session is way more flexible since you can use it to store more than just built-in types without affecting performance (you can do it with ViewState as well but you will affect performance).