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/// <summary>
/// Convert a date time object to Unix time representation.
/// </summary>
/// <param name="datetime">The datetime object to convert to Unix time stamp.</param>
/// <returns>Returns a numerical representation (Unix time) of the DateTime object.</returns>
public static long ConvertToUnixTime(DateTime datetime)
{
    DateTime sTime = new DateTime(1970, 1, 1,0,0,0,DateTimeKind.Utc);
 
    return (long)(datetime - sTime).TotalSeconds;
}

/// <summary>
/// Convert Unix time value to a DateTime object.
/// </summary>
/// <param name="unixtime">The Unix time stamp you want to convert to DateTime.</param>
/// <returns>Returns a DateTime object that represents value of the Unix time.</returns>
public static DateTime UnixTimeToDateTime(long unixtime)
{
    DateTime sTime = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
    return sTime.AddSeconds(unixtime);
}