Why is subtracting these two times in 1927 giving a strange result

0 votes

If I run the following program, which parses two date strings referencing times 1 second apart and compares them:

public static void main(String[] args) throws ParseException {
    SimpleDateFormat sf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");  
    String str3 = "1927-12-31 23:54:07";  
    String str4 = "1927-12-31 23:54:08";  
    Date sDt3 = sf.parse(str3);  
    Date sDt4 = sf.parse(str4);  
    long ld3 = sDt3.getTime() /1000;  
    long ld4 = sDt4.getTime() /1000;
    System.out.println(ld4-ld3);
}

The output is:

353

Why is ld4-ld3, not 1 (as I would expect from the one-second difference in the times), but 353?

If I change the dates to times 1 second later:

String str3 = "1927-12-31 23:54:08";  
String str4 = "1927-12-31 23:54:09";  

Then ld4-ld3 will be 1.


Java version:

java version "1.6.0_22"
Java(TM) SE Runtime Environment (build 1.6.0_22-b04)
Dynamic Code Evolution Client VM (build 0.2-b02-internal, 19.0-b04-internal, mixed mode)
Timezone(`TimeZone.getDefault()`):

sun.util.calendar.ZoneInfo[id="Asia/Shanghai",
offset=28800000,dstSavings=0,
useDaylight=false,
transitions=19,
lastRule=null]

Locale(Locale.getDefault()): zh_CN
Dec 21, 2020 in Java by Rajiv
• 8,880 points
43 views

1 answer to this question.

0 votes

In TZDB 2014f, the time of the change has moved to 1900-12-31, and it's now a mere 343 second change (so the time between t and t+1 is 344 seconds, if you see what I mean).

EDIT: To answer a question around a transition at 1900... it looks like the Java timezone implementation treats all time zones as simply being in their standard time for any instant before the start of 1900 UTC:

import java.util.TimeZone;

public class Test {
    public static void main(String[] args) throws Exception {
        long startOf1900Utc = -2208988800000L;
        for (String id : TimeZone.getAvailableIDs()) {
            TimeZone zone = TimeZone.getTimeZone(id);
            if (zone.getRawOffset() != zone.getOffset(startOf1900Utc - 1)) {
                System.out.println(id);
            }
        }
    }
}

The code above produces no output on my Windows machine. So any time zone which has any offset other than its standard one at the start of 1900 will count that as a transition. TZDB itself has some data going back earlier than that, and doesn't rely on any idea of a "fixed" standard time (which is what getRawOffset assumes to be a valid concept) so other libraries needn't introduce this artificial transition.

answered Dec 21, 2020 by Gitika
• 65,870 points

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