I was expecting this to print 1970-01-01-00:00:00, but it prints 1970-12-31-19:00:00
What am I misinterpreting about how Date is counted from? It is one year off. I am running this on Windows 7 with JDK 1.6
System.out.println(new SimpleDateFormat("YYYY-MM-dd-HH:mm:ss").format(new Date(0)));
What am I misinterpreting about how Date is counted from?
new Date(0) is January 1, 1970 in UTC. Your timezone is not UTC.
It is one year off.
No, it's not. The printed value is only 5 hours behind. Let me guess - you're somewhere in the eastern part of the US?
It's not just the time zone - it's a mixture of using the time zone and using YYYY which indicates the week year rather than the year (as of Java 7). Change it to yyyy and you'll see 1969.
The docs on week years state:
Values calculated for the WEEK_OF_YEAR field range from 1 to 53. The first week of a calendar year is the earliest seven day period starting on getFirstDayOfWeek() that contains at least getMinimalDaysInFirstWeek() days from that year. It thus depends on the values of getMinimalDaysInFirstWeek(), getFirstDayOfWeek(), and the day of the week of January 1. Weeks between week 1 of one year and week 1 of the following year (exclusive) are numbered sequentially from 2 to 52 or 53 (except for year(s) involved in the Julian-Gregorian transition).
The getFirstDayOfWeek() and getMinimalDaysInFirstWeek() values are initialized using locale-dependent resources when constructing a GregorianCalendar. The week determination is compatible with the ISO 8601 standard when getFirstDayOfWeek() is MONDAY and getMinimalDaysInFirstWeek() is 4, which values are used in locales where the standard is preferred. These values can explicitly be set by calling setFirstDayOfWeek() and setMinimalDaysInFirstWeek().
A week year is in sync with a WEEK_OF_YEAR cycle. All weeks between the first and last weeks (inclusive) have the same week year value. Therefore, the first and last days of a week year may have different calendar year values.
So the problem is that the last day of "normal year" 1969 is actually in the "week year" of 1970.
You should definitely use yyyy to avoid confusion - week years are relatively rarely useful, and can easily cause confusion, as we've seen :)
EDIT: As noted in comments, are you sure you're not using Java 7? The docs for SimpleDateFormat in 1.6 don't mention it... perhaps you're compiling in 1.6 but running in 7?
To round up the correct answers about YYYY vs. yyyy and about timezone handling, here is the code that produces the output you expected:
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd-HH:mm:ss");
sdf.setTimeZone(TimeZone.getTimeZone("UTC"));
System.out.println(sdf.format(new Date(0))); //1970-01-01-00:00:00
I suggest that you use java.time, the modern Java date and time API, for your date and time work. On Java 1.6 and 1.7 this happens through the backport of java.time to those Java version, called the ThreeTen Backport (links at the bottom).
As has been discussed in the comments, you need to decide on a time zone for interpreting 0 milliseconds after the epoch into a date and time of day. Like one such comment I am picking America/New_York time zone as an example. Insert your own.
I am using these declarations:
private static final ZoneId ZONE = ZoneId.of("America/New_York");
private static final DateTimeFormatter FORMATTER
= DateTimeFormatter.ofPattern("uuuu-MM-dd-HH:mm:ss", Locale.ROOT);
The rest is straightforward (when you know how):
String epochString = Instant.EPOCH.atZone(ZONE).format(FORMATTER);
System.out.println(epochString);
And output is:
1969-12-31-19:00:00
As has also been said already, at the epoch of 1970-01-01 UTC it is still December 31, 1969 on the western half of the globe including the Americas and a great part of the Pacific Ocean.
Pattern letter uuuu? With java.time you may use yyyy for year of era (always positive) or uuuu for a signed year. The latter is usually recommended so that you are sure to see the difference in case you (expectedly or not) come across a year from before the common era. The question uuuu versus yyyy in DateTimeFormatter formatting pattern codes in Java? linked to below covers the difference extensively.
java.time just requires at least Java 6.
org.threeten.bp with subpackages.java.time was first described.java.time to Java 6 and 7 (ThreeTen for JSR-310).uuuu versus yyyy in DateTimeFormatter formatting pattern codes in Java?
It probably has relation with your timezone. http://download.oracle.com/javase/1.4.2/docs/api/java/util/TimeZone.html
Just do * 1000L
System.out.println(new SimpleDateFormat("YYYY-MM-dd-HH:mm:ss").format(new Date(0) * 1000L));
