I wanted to figure out the importance of that value you found, so I went and did some further research. It turns out the greatest date you can create in Javascript is Date(8640000000000000). Interesting!
From the ECMAScript Spec [1]:
The actual range of times supported by ECMAScript Date objects is slightly smaller: exactly –100,000,000 days to 100,000,000 days measured relative to midnight at the beginning of 01 January, 1970 UTC. This gives a range of 8,640,000,000,000,000 milliseconds to either side of 01 January, 1970 UTC.
From the ECMAScript Spec [1]:
The actual range of times supported by ECMAScript Date objects is slightly smaller: exactly –100,000,000 days to 100,000,000 days measured relative to midnight at the beginning of 01 January, 1970 UTC. This gives a range of 8,640,000,000,000,000 milliseconds to either side of 01 January, 1970 UTC.
[1] http://ecma-international.org/ecma-262/5.1/#sec-15.9.1.1