/// <summary>
/// method for converting a UNIX timestamp to a regular
/// System.DateTime value (and also to the current local time)
/// </summary>
/// <param name="timestamp">value to be converted</param>
/// <returns>converted DateTime in string format</returns>
private static DateTime ConvertTimestamp(double timestamp)
{
    //create a new DateTime value based on the Unix Epoch
    DateTime converted = new DateTime(1970, 1, 1, 0, 0, 0, 0);

    //add the timestamp to the value
    DateTime newDateTime = converted.AddSeconds(timestamp);

    //return the value in string format
    return newDateTime.ToLocalTime();
}

/// <summary>
/// method for converting a System.DateTime value to a UNIX Timestamp
/// </summary>
/// <param name="value">date to convert</param>
/// <returns></returns>
private double ConvertToTimestamp(DateTime value)
{
    //create Timespan by subtracting the value provided from
    //the Unix Epoch
    TimeSpan span = (value - new DateTime(1970, 1, 1, 0, 0, 0, 0).ToLocalTime());

    //return the total seconds (which is a UNIX timestamp)
    return (double)span.TotalSeconds;
}