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);