TimeExt.cs

public static class TimeExt
{
    private static readonly DateTime Epoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);

    public static long ToUnixTimeStamp(this DateTime dateTime)
    {
        DateTime ut = dateTime.ToUniversalTime();
        var unixTimeStamp = (long)(ut - Epoch).TotalMilliseconds;
        return unixTimeStamp;
    }

    public static DateTime ToDateTime(this long unixTimeStamp)
    {
        DateTime dateTime = Epoch.AddMilliseconds(unixTimeStamp);
        return new DateTime(dateTime.Ticks, DateTimeKind.Utc).ToLocalTime();
    }

    public static string ToBase64(this string s)
    {
        return Convert.ToBase64String(Encoding.UTF8.GetBytes(s));
    }
}