mongodb ISODate problems

Question

I am using java(IDE is eclipse) to query on mongodb. Below is my java code:

DBObject query = new BasicDBObject();
ObjectId id =new ObjectId("529f280b90ee58cb7732c2b8");
query.put("_id", id);
DBCursor cursor = collection.find(query);
while(cursor.hasNext()) {
    DBObject object = (DBObject)(cursor.next());
    System.out.println(object.get("_id"));
    System.out.println(object.get("createDate"));
}

Problems happened in the createDate whose type is ISODate and value is ISODate("2013-10-21T01:34:04.808Z"), but the println result of my code is 'Mon Oct 21 **09**:34:04 CST 2013', the hour has changed from 01 to 09. I don't know what happened!

Can anybody help?

Solution

The hour did not change. You must be in China, given the "CST" in your example and the 8 hour difference. If you interpret "CST" as "China Standard Time" (rather than Central Standard Time in the US), then you have a time zone that is 8 hours ahead of UTC/GMT. So when ti is 1 AM UTC/GMT, at the vary same moment the clock on the wall in Taipei will read "9 AM".

Minor point: Those three-letter codes for time zones are obsolete and should be avoided. They are neither standardized nor unique. Use proper time zone names.

Major point: The problem lies in how you extract a value from MongoDB that represents a date-time.

I don't know MongoDB, and their doc is confusing, so I can't help you much further. If you can retrieve an ISO 8601 string as seen in your first example, that is much preferable to the format of your second example.

If you want to work with the date-time value in Java, you can feed an ISO 8601 string directly to a DateTime constructor in Joda-Time 2.3.

DateTime dateTime = new DateTime( "2013-10-21T01:34:04.808Z" );

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Update

This doc says that the Java driver for MongoDB will give you a java.util.Date object. That explains your problem. The java.util.Date & Calendar classes bundled with Java are notoriously bad. One problem is that while a Date instance has no time zone, its toString() method uses the JVM's default time zone to render a string. And Date's toString method uses that terrible ambiguous format.

You should avoid using java.util.Date & Calendar classes. For now use the Joda-Time library. In Java 8, you can use the new java.time.* classes.

You can convert back and forth between java.util.Date and Joda-Time. Pass a Date instance to Joda-Time constructor. To go back, call Joda-Time toDate() format.

Note that while a java.util.Date has no time zone information within it, in contrast a DateTime object does have a time zone assigned. If you want UTC/GMT, specify DateTimeZone.UTC.

Your code should look more like:

java.util.Date date = object.get("createDate");
DateTime createDateTime = new DateTime( date, DateTimeZone.forId( "Asia/Manila" ) );
System.out.println( createDateTime );
… do some work …
java.util.Date dateGoingBackToMongoDB = createDateTime.toDate();