为什么UTC时区为Java中的System.currentTimeMillis提前了时间？

Question

I am getting current time from RubyOnRails webservice in Unix Timestamp format(ie. in seconds from 1 Jan 1970), the timezone on server is UTC .

In Java I am trying to convert local current time to UTC time . But every time it is giving 6+ minutes ahead time. I want to get the difference of UTC current time and the time returned from service. My Java code is -

SimpleDateFormat sdf = new SimpleDateFormat("dd-MM-yyyy hh:mm:ss");
Date utc_current = new Date(System.currentTimeMillis());
sdf.setTimeZone(TimeZone.getTimeZone("UTC"));
long serverTime = 1424936268000L;
long resTime = sdf.getCalendar().getTimeInMillis() - serverTime;
System.out.println("Time Diff : " + resTime);

Where serverTime is the time I am getting from webservice. And the value for resTime shows negative value which is approx 6+ minutes.

So my question is why UTC timezone giving ahead time for System.currentTimeMillis?

Please help me to resolve this issue.

Answer 1

In contrast to the assumption in a comment of of @JB Nizet the expressions sdf.getCalendar().getTimeInMillis() and System.currentTimeMillis() are not equivalent. Proof:

SimpleDateFormat f = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss");
System.out.println("date via System.currentTimeMillis()=" + f.format(utc_current));
System.out.println("date via sdf.getCalendar()=" + f.format(new Date(resTime)));

Output:

date via System.currentTimeMillis()=2015-02-26T12:19:09
date via sdf.getCalendar()=1889-12-31T04:41:21

If you carefully study the source code of SimpleDateFormat and DateFormat you will find within the initialization part code like:

private void initializeDefaultCentury() {
    calendar.setTimeInMillis(System.currentTimeMillis());
    calendar.add( Calendar.YEAR, -80 );
    parseAmbiguousDatesAsAfter(calendar.getTime());
}

The conclusion is to strictly avoid the method getCalendar() on your DateFormat -object. It is only used as intermediate mutable object for internal format and parse processing. It is hard to say what you will really get as time this way. Instead use directly System.currentTimeMillis() to compare your local time with server time.

Another problem is the pattern you use. "dd-MM-yyyy hh:mm:ss" is probably not correct because it uses the clock hour of half day in range 1-12 but the information for am/pm is missing. Use better the pattern symbol HH. Check the documentation of webservice for the right format.

Answer 2

Make sure the the clock on the server and on the client machine are synchronized. The 6 minutes could simply be an offset between the two.