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

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")
    }