using System;

​

namespace Foo.Bar

{

    public class Program

    {

        public static void Main()

        {

            var date = new DateTime(2021, 5, 18, 7, 15, 20);

            var retryAfter = RetryAfterFrom(date, TimeSpan.FromSeconds(300));

            Console.WriteLine(retryAfter);

        }

        public static string RetryAfterFrom(DateTime timestamp, TimeSpan period)

        {

            var diff = (timestamp + period) - DateTime.UtcNow;

            var seconds = Math.Max(diff.TotalSeconds, 1);

​

            return $"{seconds:F0}";

        }

    }

}