date in MongoDB: when inserting Date objects into Mongo database, the date becomes 1 day earlier than itself

Question

My date string format is like this: Jan 2, 2012 After the Instant.parse() method, instant instance becomes the date of Jan 1, 2012, which is 1 day earlier, why? If the original date string is jan 1, 2012, the Instant will be the date of Dec 31, 2011.

String dateString="Jan 1, 2012";
Instant instant = Instant.parse(dateString, new DateTimeFormatterBuilder()
.appendMonthOfYearShortText()
.appendLiteral(" ")
.appendDayOfMonth(1)
.appendLiteral(", ")
.appendYear(4, 4)
.toFormatter());

DateTime dateTime = new DateTime(instant);
Date date = new Date(dateTime.getMillis());

document.append("time", new Date(dateTime.getMillis()));
tagsDbCollection.insert(document);

---

I'm using MongoDB to store these dates. I've tested and it shows when formatting date string->instant there's no mistake. But when I insert this Date type object into MongoDB, the date string in the MongoDB becomes 1 day earlier., why?

In MongoDB:

 /* 0 */
    {
      "_id" : ObjectId("50221a40da74d74053abb445"),
      "time" : ISODate("2011-12-31T14:00:00Z")
    }

Answer 1

final String dateString = "Jan 2, 2012";
final DateTimeFormatter dtf = new DateTimeFormatterBuilder().appendMonthOfYearShortText().appendLiteral(" ").appendDayOfMonth(1).appendLiteral(", ").appendYear(4, 4).toFormatter();
final DateTime jodaDate = dtf.parseDateTime(dateString);
System.out.println(jodaDate);
final Date javaDate = new Date(jodaDate.getMillis());
System.out.println(javaDate); 

Output is

2012-01-02T00:00:00.000+02:00
Mon Jan 02 00:00:00 EET 2012  

Next for:

final String dateString = "Jan 1, 2012";

output is:

2012-01-01T00:00:00.000+02:00
Sun Jan 01 00:00:00 EET 2012

Answer 2

Mongo stores its Dates in milliseconds since the Unix epoch.

See: http://www.mongodb.org/display/DOCS/Dates

So you dont have any time zone. But, if you use the console the .js parser is converting the UTC Dates into your current system time zone settings.

You can test that:

• Create an Entity with some Date data.
• then querying it via the console. (use String())
• then exit the console and reconfigure the system time zone (debian/ubuntu: sudo dpkg-reconfigure tzdata )
• then enter the console again and query your old data => you get the same UTC but different toString() outputs