using System;

using System.Collections.Generic;

using System.Text.Json;

using System.Linq;

public class Program

{

    private static List<ScheduleTime> listOfSchedulers = null;

    private static IList<FormattedScheduleTime> formattedList = new List<FormattedScheduleTime>();

    public static void Main()

    {

        listOfSchedulers = new List<ScheduleTime>(){

            new ScheduleTime(){

                StartDate = DateTime.Parse("2021-02-10T13:35"),

                EndDate = DateTime.Parse("2021-02-18T15:05"),

                DaysOfWeek = "0,3,6"

            },

            new ScheduleTime(){

                StartDate = DateTime.Parse("2021-02-05T11:05"),

                EndDate = DateTime.Parse("2021-02-09T12:25"),

                DaysOfWeek = null

            },

        };

        listOfSchedulers.ForEach(item => {

            while(item.StartDate <= item.EndDate){

                if(item.DaysOfWeek != null){

                    var splittedList = item.DaysOfWeek.Split(",");

                    for(int i =0; i <splittedList.Length; i++){

                        if((DayOfWeek)Int32.Parse(splittedList[i]) == item.StartDate.DayOfWeek){

                            var record = new FormattedScheduleTime(){

                                DayOfWeek = item.StartDate.DayOfWeek.ToString(),

                                Date = item.StartDate.ToString("yyyy-MM-dd"),

                                Time = $"{item.StartDate.ToString("HH:mm")} - {item.EndDate.ToString("HH:mm")}"

                            };

                            formattedList.Add(record);

                        }

                    }

                }else{

                    var record = new FormattedScheduleTime(){

                                DayOfWeek = item.StartDate.DayOfWeek.ToString(),

                                Date = item.StartDate.ToString("yyyy-MM-dd"),

                                Time = $"{item.StartDate.ToString("HH:mm")} - {item.EndDate.ToString("HH:mm")}"

                            };

                            formattedList.Add(record);

                }

                item.StartDate = item.StartDate.AddDays(1);

            }

        });

        Console.Write(JsonSerializer.Serialize(formattedList.OrderBy(i => DateTime.Parse(i.Date))));

    }

    private class ScheduleTime {

        public DateTime StartDate {get; set;}

        public DateTime EndDate {get; set;}

        public string DaysOfWeek {get; set;}

    }

    private class FormattedScheduleTime {

        public string DayOfWeek {get; set;}

        public string Date {get;set;}

        public string Time {get; set;}

    }

}

​

​