You'll need the help of an ASP.NET WebService / MVC / Web Form or the like that can pass back to your JavaScript the current time; either through page code itself, or through an AJAX request.
Here's some C# code that translates a ticks integer to a microtime timestamp:
DateTime someDate = new DateTime(635556672000000000);
Int32 unixTimestamp = (Int32)(someDate.Subtract(new DateTime(1970, 1, 1))).TotalSeconds;
unixTimestamp = unixTimestamp * 1000;
Once you've set something up so that you can get the value of unixTimestamp out of an ASP.NET resource, just call it via jQuery or the like. For example, supposing you create an ASP.NET web handler, named time.ashx, that spits out the microtime stamp as plaintext:
var myTimestamp = new Date();
$.get("http://www.example.com/time.ashx", function() { }).done(function(data) {
myTimestamp = new Date(data);
});
Or something similar. Point is: ASP.NET helper to convert ticks to microtime.