The JavaScript Date type's origin is the Unix epoch: midnight on 1 January 1970.
The .NET DateTime type's origin is midnight on 1 January 0001.
You can translate a JavaScript Date object to .NET ticks as follows:
var yourDate = new Date(); // for example
// the number of .net ticks at the unix epoch
var epochTicks = 621355968000000000;
// there are 10000 .net ticks per millisecond
var ticksPerMillisecond = 10000;
// calculate the total number of .net ticks for your date
var yourTicks = epochTicks + (yourDate.getTime() * ticksPerMillisecond);
The DateTime value type represents dates and times with values ranging from 00:00:00 (midnight), January 1, 0001 Anno Domini (Common Era) through 11:59:59 P.M., December 31, 9999 A.D. (C.E.) in the Gregorian calendar.
621355968000000000 is the value of ticks from midnight Jan 1 01 CE to midnight Jan 1 1970
So, in .NET
Console.Write(new DateTime(621355968000000000))
// output 1/1/1970 12:00:00 AM
Hence to convert javascript time to .Net ticks
var currentTime = new Date().getTime();
// 10,000 ticks in 1 millisecond
// jsTicks is number of ticks from midnight Jan 1, 1970
var jsTicks = currentTime * 10000;
// add 621355968000000000 to jsTicks
// netTicks is number of ticks from midnight Jan 1, 01 CE
var netTicks = jsTicks + 621355968000000000;
Now, in .NET
Console.Write(new DateTime(netTicks))
// output current time