DateSlider value in javascript callback

Hi,
I tried to convert a DateSlider value (original python’s datetime.datetime object) to a Javascript Date, then I need to convert it back to string as follows:

var date = new Date(cb_obj.value*1000);

``

But the result Date was not correct. What is the correct way to do it? or how can I get a string from cb_obj.value in my CustomJS callback.

Thanks

Bokeh 0.12.14 server on Ubuntu 16.04 and Python 3.6.3, Chrome client