Epoch

public static class UnixTimestamp
{
    private static readonly DateTime Epoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
    
    public static double Convert(this DateTime dateTime)
    {
        return dateTime.Subtract(Epoch).TotalMilliseconds;
    }

    public static DateTime Convert(this double offset)
    {
        return Epoch.AddMilliseconds(offset);
    }
}