Getting inconsistent results with SimpleDateFormatter

Question

I am running below snippet and I am getting inconsistent resluts

SimpleDateFormat inputFormat = new SimpleDateFormat("yyyy-M-dd HH:mm:ss.SSS");
         Date date;
         date = inputFormat.parse("2020-6-30 11:45:45. 123");
     SimpleDateFormat  outputFormat = new SimpleDateFormat("MM-dd-yy hh:mm:ss. SSS");
         System.out.println(outputFormat.format(date));//06-30-20 11:45:45. 123 is the output

Snippet 2:

SimpleDateFormat inputFormat = new SimpleDateFormat("yyyy-M-dd HH:mm:ss.SSSSSS");
         Date date;
         date = inputFormat.parse("2020-6-30 11:45:45. 123456");
     SimpleDateFormat  outputFormat = new SimpleDateFormat("MM-dd-yy hh:mm:ss. SSSSSS");
         System.out.println(outputFormat.format(date));//06-30-20 11:47:48. 000456 is output

snippet 3:

SimpleDateFormat inputFormat = new SimpleDateFormat("yyyy-M-dd HH:mm:ss.SSSSSSSSS");
         Date date;
         date = inputFormat.parse("2020-6-30 11:45:45. 123456789");
     SimpleDateFormat  outputFormat = new SimpleDateFormat("MM-dd-yy hh:mm:ss. SSSSSSSSS");
         System.out.println(outputFormat.format(date));//07-01-20 10:03:21. 000000789 is output

I believe the fraction seconds should be same before and after conversion. How can I achieve consistent results

Please read the comments and answers of thie question: https://stackoverflow.com/questions/19223171/java-util-date-format-ssssss-if-not-microseconds-what-are-the-last-3-digits — TomStroemer, Jul 30 '20 at 07:12
`S` in `SimpleDateFormat` represents millisecond. How many milliseconds can you fit into one second? How many seconds do you get if you have 123456789 milliseconds? When you look at your results you can see that your output changes completely, giving you a few extra days in the last snippet. — Amongalen, Jul 30 '20 at 07:13
On a sidenote - for a start I'd move away from the ancient `Date` type and use something from `java.time` package instead. A whole new package for handling date/time was created for a reason. — Amongalen, Jul 30 '20 at 07:26

score 3 · Accepted Answer · answered Jul 30 '20 at 07:17

The first one is correct, because your milliseconds are between 0 - 999. The second and third one are technically also correct, except your 123456 milliseconds are converted in 123 seconds + 456 milliseconds, which results in a time of +2 mins 03 secs.

So instead of trying to fix the output of simple date format, you have to fix the way you handle the input. Simple date format can not parse anything smaller than milliseconds. If you provide it with a number bigger than 999 milliseconds, it affects the seconds and so on...

score 0 · Answer 2 · answered Jul 30 '20 at 07:27

Since you use java 8, I would VERY STRONGLY recommend abandoning horrible and outdated (no pun intended) SimpleDateFormat. It is not thread-safe, and the whole Date processing before java 8 needed to be rewritten badly. That's why there were external library alternatives that dealt with Date processing. (One well-known example is Joda time library). In Java 8 the Date processing has been majorly redesigned and re-written. So, please look up the packages java.time and all related ones java.time.*. Date formatting is done by DateTimeFormatter class. The recommendation is to switch to new Date processing packages. Note that all existing bugs will not be fixed for the old classes such as SimpleDateFormat

Getting inconsistent results with SimpleDateFormatter

2 Answers2