SimpleDateFormat is not parsing the milliseconds correctly

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开发者_如何学Go 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?

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

I'm not entirely sure, but the JavaDoc states this:

For parsing, the number of pattern letters is ignored unless it's needed to separate two adjacent fields.

This indicates that the milliseconds from 2011-08-23 14:57:26.9 would be parsed as 9 instead of 900. Adding the trailing zeros might work: 2011-08-23 14:57:26.900.

I'd suggest using Joda-Time. It handles these situations properly. In the following example, the milliseconds are correctly parsed as 200ms.

import org.joda.time.DateTime;
import org.joda.time.format.DateTimeFormat;
import org.joda.time.format.DateTimeFormatter;

public class ParseMillis {

  public static void main(String[] args) {
    String s = "00:00:01.2";
    DateTimeFormatter format = DateTimeFormat.forPattern("HH:mm:ss.S");
    DateTime dateTime = format.parseDateTime(s);
    System.out.println(dateTime.getMillisOfSecond());
  }
}

I had the same problem with too accurate time from my logfiles with 6 digit milliseconds. Parsing Time gave up to 16 minutes difference! WTF?

16-JAN-12 04.00.00.999999 PM GMT --> 16 Jan 2012 04:16:39 GMT

Changing the number of digits reduced the erroneous difference and thanks to this thread I could identify the problem:

16-JAN-12 04.00.00.99999 PM GMT --> 16 Jan 2012 04:01:39 GMT
16-JAN-12 04.00.00.9999 PM GMT --> 16 Jan 2012 04:00:09 GMT
16-JAN-12 04.00.00.999 PM GMT --> 16 Jan 2012 04:00:00 GMT

As SimpleDateFormat internally handles only 3 digits I removed the unnecessary with a small regex (ignoring round-off errors, working for 1 up to n digits):

str = str.replaceAll("(\\.[0-9]{3})[0-9]*( [AP]M)", "$1$2");

Thanks to @Peter Lawrey for your answer, prevented me going insane :-)