using System;

public class Program

{

    private static long lrsn_to_ts(long ifi_ts) {

        const long APR_EPOCH_TO_IFI_EPOCH = 2208988800000000L; // micro-seconds between APR epoch (1970-01-01) and IBM IFI epoch (1900-01-01).

        return (ifi_ts >> 8) - APR_EPOCH_TO_IFI_EPOCH;

    }

​

    public static void Main()

    {

        var lrsn = 0xDD1B14B6BD451A8L;

        var tsInMicroSecs = lrsn_to_ts(lrsn);

        var tsInSecs = tsInMicroSecs/1000000;

        var microSecFrac = tsInMicroSecs - 1000000.0 * tsInSecs;

        var ts1 = DateTimeOffset.FromUnixTimeSeconds(tsInSecs).AddMicroseconds(microSecFrac);

        var ts = ts1.ToString("yyyy-MM-dd'T'HH:mm:ss.ffffffZ");

        Console.WriteLine($"lrsn: [00{lrsn:X}000000000] => ts: {ts}");

    }

}