Was done in the base year of the Java calendar week


I have a java app that I developed. There is a function to register items in it, and the registered date is also registered in the DB. I registered on 2019/12/29, but when I looked at the data registered in the DB, the registered date was 2020/12/29. I wondered why, and investigated the cause, so I will summarize it here as a reminder.


As a result, the registered date is registered as a character string type in the yyyy / MM / dd format. Therefore, the date type was converted to the character string type in the yyyy / MM / dd format with Java. Use Java's SimpleDateFormat for conversion. By defining the year part with YYYY, the year is treated as the base year of the calendar week </ b>, and 2019/12/29 is converted to 2020/12/29 and registered in the DB.

What is the base year of a calendar week?

The base year of a calendar week is the idea that "the week to which 1/1 belongs is treated as the next year even if it is December." The expression is poor and difficult to understand. Excuse me.

It looks like this on a calendar sdf_001.png December 29-31 is treated as 2020 because the week with a yellow background includes the day of 2020. That is.

  • 12/28 is treated as 2019.


By the way, the code looked like this.


//* Execution date is 2019/12/29
SimpleDateFormat sdf = new SimpleDAteFormat("YYYY/MM/dd");
Calender cal = Calender.getInstance();

String yyyymmdd = sdf.format(cal.getTime());    //2020 at this timing/12/It becomes 29.

To handle it normally, just change YYYY to yyyy.

That was a terrible mistake.

at the end

I did java after a long time, so I made such a terrible mistake. Until then, I was touching python, and in the string conversion of the date of python, % Y is used for the year, so I think I confused it and specified the uppercase Y. You should be careful.


By the way, as for the code above, you can only get the current date and time with Date. It is said that it reveals its low coding ability. : dizzy_face:

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