using System;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        ListTimespans(new DateTime(2020, 1, 1), new DateTime(2020, 01, 31), 7, 10).Dump();

    }

    public static IEnumerable<(DateTime start, DateTime end)> ListTimespans(DateTime startDate, DateTime endDate, int minDays, int maxDays)

    {

        // Loop through all of the possible starts. The first day we can start on is startDate, and the last day we can

        // start on is endDate - minDays (which gives us a span minDays long ending on endDate)

        for (var start = startDate; start <= endDate.AddDays(-minDays); start = start.AddDays(1))

        {

            // For each of these starts, loop through the possible end dates. There are two limits to the end date:

            // either hit the maximum number of days for a span (start.AddDays(maxDays)), or we hit the end date.

            // Loop until whichever one comes first.

            for (var end = start.AddDays(minDays); end <= Min(start.AddDays(maxDays), endDate); end = end.AddDays(1))

            {

                yield return (start, end);  

            }

        }

        DateTime Min(DateTime x, DateTime y) => x < y ? x : y;

    }

}