using System;

public class Program

{

    public static void Main()

    {

        string docId = "66f78a1f80101edc50e45bb2";

        var timestampHex = docId.Substring(0, 8);

        Console.WriteLine(timestampHex);

        int timestampDecimal = Convert.ToInt32(timestampHex, 16);

        Console.WriteLine(timestampDecimal);

        DateTime dateTime = DateTimeOffset.FromUnixTimeSeconds(timestampDecimal).UtcDateTime;

        Console.WriteLine(dateTime);

    }

}