correct-datetime-creation.cs

var secondsSinceReferenceDate = 386463600; // 2013-4-1
var referenceDate = new DateTime(2001, 1, 1);

var databaseUtcTime = refdate.AddSeconds(secondsSinceReferenceDate);

TimeZoneInfo timeZone;
try {
    timeZone = TimeZoneInfo.FindSystemTimeZoneById("Europe/Dublin");
}
catch (Exception)
{
    timeZone = TimeZoneInfo.Local;
}

var GMTTime = TimeZoneInfo.ConvertTimeFromUtc(databaseUtcTime, GMTTimeZone);

incorrect-datetime-creation.cs

var secondsSinceReferenceDate = 386463600; // 2013-4-1
var referenceDate = new DateTime(2001, 1, 1);

var validDate = referenceDate.AddSeconds(secondsSinceReferenceDate);

olson-example.cs

// On Windows Server the equivalent would be "GMT Standard Time"
TimeZoneInfo GMTTimeZone = TimeZoneInfo.FindSystemTimeZoneById("Europe/Dublin");