[JAVA] Be careful of daylight saving time when newly registering / referencing a user's birthday

1. Overview

Convert to milliseconds on the front side When sending time information to the backend API You need to consider daylight savings time.

If you convert it with a Java method, it will handle daylight saving time, This is because javascript does not handle it.

In this article, how Java handles daylight savings time I will tell you the detailed specifications of the conversion.

• Please see the following article for details on summer time. Daylight Saving Time-Wikipedia

problem

The birthday of a person born during daylight saving time An event has occurred that is displayed one day off.

Solution

1 hour during registration / reference during daylight saving time You need to add and subtract milliseconds.

2. Details

The period for introducing daylight saving time in Japan is as follows.

Date and time ⇒ milliseconds

Start of summer time

May 2nd (Monday) at 2:00 am to enter daylight saving time The time is advanced by one hour and the millisecond is the same as 3:00 am.

** Verification source **

SummerTime.java

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

public class SummerTime {
	public static void main(String[] args) throws ParseException {
		convertToMilliseconds("1948-05-02 02:00:00");
		convertToMilliseconds("1948-05-02 03:00:00");
	}

	private static void convertToMilliseconds(String dateStr) throws ParseException {
		SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
		Date date = formatter.parse(dateStr);
		System.out.println(dateStr + " -> " + date.getTime());
	}
}

** Output result **

1948-05-02 02:00:00 -> -683794800000
1948-05-02 03:00:00 -> -683794800000

The conversion at 2:00 am is as follows.

1948-05-02 02:00:00 -> -683794800000
1948-05-02 02:00:01 -> -683794799000
1948-05-02 02:00:02 -> -683794798000
・
・
・
1948-05-02 02:59:58 -> -683791202000
1948-05-02 02:59:59 -> -683791201000
1948-05-02 03:00:00 -> -683794800000

From 2:59:59:999 to 3:00 am I understand the specifications to be corrected. However, there is no 2:00 am during daylight saving time. It is not expected that the millisecond conversion between 2 and 3 o'clock will be used.

End of summer time

To get out of daylight saving time at 1:00 am on Monday, September 11 The time must be restored by one hour.

1948-09-11 00:59:59 -> -672397201000
1948-09-11 01:00:00 -> -672393600000

The difference is 1 second in the date and time notation, In milliseconds, it is corrected to 360 1000 milliseconds.

Milliseconds ⇒ date and time

Start of summer time

Now let's switch back to the date and time notation from milliseconds.

** Verification source **

SummerTime.java

private static void convertToTimeFormat(Long milliSeconds) throws ParseException {
	SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
	String dateStr = formatter.format(milliSeconds);
	System.out.println(milliSeconds + " -> " + dateStr);
}

** Output result **

-683794801000 -> 1948-05-02 01:59:59
-683794800000 -> 1948-05-02 03:00:00
-683794799000 -> 1948-05-02 03:00:01

End of summer time

-672397201000 -> 1948-09-11 00:59:59
-672397200000 -> 1948-09-11 01:00:00
-672397199000 -> 1948-09-11 01:00:01
・
・
・
-672393601000 -> 1948-09-11 01:59:59
-672393600000 -> 1948-09-11 01:00:00
-672393599000 -> 1948-09-11 01:00:01

The specifications are such that the 1 am level comes twice.

3. Remedy

When converting at the front, correct the milliseconds of the corresponding period Fixed to send time information to API.

dateUtil.ts

    /**
     *Get time considering daylight saving time (converted when displaying from API to front)
     */
    export function getSummerTimeDate(birthDate){
        let birth = Number(birthDate)
        if ((birth >= -683794800000 && birth <= -672393600001)
            || (birth >= -654764400000 && birth <= -640944000001)
            || (birth >= -620290800000 && birth <= -609494400001)
            || (birth >= -588841200000 && birth <= -578044800001)) {
            birth += 3600000
        }
        return birth
    }

    /**
     *Substitute time considering daylight saving time (converted when registering from front to API)
     */
    export function setSummerTimeDate(birthDate){
        let birth = Number(birthDate)
        if ((birth >= -683791200000 && birth <= -672390000001)
            || (birth >= -654760800000 && birth <= -640940400001)
            || (birth >= -620287200000 && birth <= -609490800001)
            || (birth >= -588837600000 && birth <= -578041200001)) {
            birth -= 3600000
        }
        return birth
    }

4. Summary

I think it is better to consider whether to handle the time of daylight saving time introduction at the stage of considering the specifications. Also, I think we should consider another way to avoid it, such as using UTC for the time zone.

5. Reference

Japanese Daylight Saving Time and Java About the real summer time