using System;

public class Program

{

    public static readonly DateTime UnixEpoch = new DateTime(1970, 1, 1, 0, 0, 0, 0, DateTimeKind.Utc);

    public static void Main()

    {

        var tokenValidFor = TimeSpan.FromSeconds(2);

        var now = DateTime.UtcNow;

        var expiry = now.Add(tokenValidFor);

        Console.WriteLine(GetEpochDateTimeAsInt(now)); // nbf

        Console.WriteLine(GetEpochDateTimeAsInt(expiry)); // exp

    }

    public static long GetEpochDateTimeAsInt(DateTime datetime)

    {

        DateTime dateTime = datetime;

        if (datetime.Kind != DateTimeKind.Utc)

            dateTime = datetime.ToUniversalTime();

        if (dateTime.ToUniversalTime() <= UnixEpoch)

            return 0;

        return (long)(dateTime - UnixEpoch).TotalSeconds;

    }

}