java - SimpleDateFormat is not parsing the milliseconds correctly

Question

Background:

In my database table, I have two timestamps

timeStamp1 = 2011-08-23 14:57:26.662
timeStamp2 = 2011-08-23 14:57:26.9

When I do an "ORDER BY TIMESTAMP ASC", timeStamp2 is considered as the greater timestamp(which is correct).

Requirement: I need to get the difference of these timestamps (timeStamp2 - timeStamp1)

My implementation:

public static String timeDifference(String now, String prev) {
    try {
        final Date currentParsed = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS").parse(now);
        final Date previousParsed = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS").parse(prev);
        long difference = currentParsed.getTime() - previousParsed.getTime();
        return "" + difference;
    } catch (ParseException e) {
        return "Unknown";
    }
}

The answer should have been 238ms, but the value that is returned is -653ms. I'm not sure what I'm doing wrong. Any suggestions?

Answer 1

The format you are parsing and the format uses doesn't match. You expect a three digit field and are only providing one digits. It takes 9 and assumes you mean 009 when what you want is 900. Date formats are complicated and when you prove dates in a different format it may parse them differently to you.

The documentation says S means the number of milli-seconds and the number in that field is 9, so it is behaving correctly.

---

EDIT: This example may help

final SimpleDateFormat ss_SSS = new SimpleDateFormat("ss.SSS");
ss_SSS.setTimeZone(TimeZone.getTimeZone("GMT"));
for (String text : "0.9, 0.456, 0.123456".split(", ")) {
  System.out.println(text + " parsed as "ss.SSS" is "
      + ss_SSS.parse(text).getTime() + " millis");
}

prints

0.9 parsed as "ss.SSS" is 9 millis
0.456 parsed as "ss.SSS" is 456 millis
0.123456 parsed as "ss.SSS" is 123456 millis