C# DateTime 시간, 날짜 다루기 - DateTime.Parse(), TimeSpan.FromSecond(), DateTime.Add(), DayOfWeek

DateTime

immutable(불변의) object that contains both date and time information.

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DateTime 특징

• 한 번 DateTime이 생성되면 값이 절대 변하지 않는다. DateTime을 modify하려고 하면 return new DateTime
• textual representaion은 dependent on the culture
  • en-US(America English) : "3/28/19 2:30:59 PM"

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DateTime.Parse() : string을 DateTime으로 변환

using System;

static class Appointment
{
	// string -> DateTime
    public static DateTime Schedule(string appointmentDateDescription)
    {
        return DateTime.Parse(appointmentDateDescription);
    }
	
    // DateTime.Now
    public static bool HasPassed(DateTime appointmentDate)
    {
        // DateTime이 과거인지
        if(DateTime.Now > appointmentDate){
            return true;
        }else{
            return false;
        }
    }
	
    // DateTime.Hour
    public static bool IsAfternoonAppointment(DateTime appointmentDate)
    {
        // 오후(12시 이후 18시 미만)인지
        if(appointmentDate.Hour >= 12 && appointmentDate.Hour < 18){
            return true;
        }else{
            return false;
        }
    }
	
    // DateTime -> String 변환
    public static string Description(DateTime appointmentDate)
    {
        return $"You have an appointment on {appointmentDate}.";
    }

    public static DateTime AnniversaryDate()
    {
        // 2019/9/15 일을 반환하라
        return new DateTime(2025,9,15,0,0,0);
    }
}

---

TimeSpan.FromSeconds()

DateTime.Add()

using System;

public static class Gigasecond
{
    // 1 gigasecond = 1000000000 seconds

    //born in 2015/01/24 22:00
    // + 1 gigasecond
    //2046/10/02 11:46:40pm

    // 1000000000 seconds
    // 16,666,666 minutes 40 seconds
    // 277,777 hours 46 minutes 40 seconds
    // 11,574 days 1 hours 46 minutes 40 seconds

    // 
    
    public static DateTime Add(DateTime moment)
    {
          TimeSpan gigasecond = TimeSpan.FromSeconds(1_000_000_000);
        return moment.Add(gigasecond);
    }
}

using System;

public static class Gigasecond
{
    public static DateTime Add(DateTime birthDate) => birthDate.AddSeconds(1e9);
}

---

DateTime생성 및 DayOfWeek

DateTime firstDayOfMonth = new DateTime(year, month, 1);
DateTime targetDate;

case Schedule.Teenth:
       for (int day = 13; day <= 19; day++)
       {
             targetDate = new DateTime(year, month, day);
             // DayOfWeek(요일)
             if (targetDate.DayOfWeek == dayOfWeek)
                    return targetDate;
       }

DateTime firstDayOfMonth = new DateTime(year, month, 1);
DateTime targetDate;

case Schedule.First:
      targetDate = firstDayOfMonth;
      while (targetDate.DayOfWeek != dayOfWeek)
             targetDate = targetDate.AddDays(1);
      		return targetDate;

https://exercism.org/tracks/csharp/exercises/meetup/iterations