Result of toJSON() on a date is different between IE8 and IE9+

I'm doing a conversion from Date to string and back for using in sessionStorage. so I first do this:

sessionStorage.currentDate = myDate.toJSON();

and then I do this:

if (sessionStorage.currentDate ) {
    myDate = new Date(sessionStorage.currentDate);
}

The problem is that the myDate.toJSON() function in IE9+ returns "2013-05-06T22:00:00.000Z" but in IE8 it returns "2013-05-06T22:00:00Z" missing the decimal part at the end.

The fact is that in IE8 is failing the subsequent re-conversion into a date (the result from new Date(sessionStorage.currentDate) is NaN)

Any idea why this is happening and how to make this code work for IE8+?

Update:

I tried to replace the string in debug, and it turns out that none of the 2 strings works. So it actually seems to be a problem of the new Date(sessionStorage.currentDate) not recognizing the format (which is UTC)