JsDateHelper.cs

public class JsDateHelper : IJsDateHelper
{
    public DateTime FromUnixTime(double unixTime)
    {
        var epoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
        return epoch.AddMilliseconds(unixTime);
    }

    public long ToUnixTime(DateTime date)
    {
        var epoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
        return Convert.ToInt64((date - epoch).TotalMilliseconds);
    }
}

main.js

var birthday = new Date(1994, 12, 10);
var time = birthday.getTime(); // 789696000000
var date2 = new Date(time); // Same as birthday