What's the difference between these two ways of creating a DateTime from a LocalDateTime?

Binney :

Our application uses jodatime to handle times, and (for API formatting reasons) we store times in a model class which looks a bit like this:

class Event {
    private LocalDateTime localTime;
    private DateTimeZone timeZone;

    public DateTime getTime() {
        return localStopTime.toDateTime(timeZone);
    }

    public void setTime(DateTime value) {
        this.localTime = value.toLocalDateTime();
        this.timeZone = value.getZone();
    }
    // ...more boilerplate
}

Further downstream I noticed we were getting a different time out than we were setting. I figured we were converting the fields back to a DateTime wrong, since the local fields seem to have the right values.

On a whim I tried changing the getter and now it works, but I have no idea why:

    public DateTime getTime() {
        return localStopTime.toDateTime().withZone(timeZone);
    }

The joda documentation is a bit tight-lipped about how it carries out the toDateTime() call; it says it "uses" a certain timezone somehow but that's it.

Can anyone explain to me what the difference is between

return localStopTime.toDateTime(timeZone);

and

return localStopTime.toDateTime().withZone(timeZone);

?

Thanks in advance!

Edit: I've figured it out - I was using "Etc/GMT" as my time zone and that didn't take into account daylight savings. Have marked Marco's answer as correct

Marco Marchetti :

The difference between those two is the next one, you use withZone() to: (as JavaDocs says)

Returns a copy of this datetime with a different time zone, preserving the millisecond instant.

Also, the JavaDocs provides a good example:

This method is useful for finding the local time in another timezone. For example, if this instant holds 12:30 in Europe/London, the result from this method with Europe/Paris would be 13:30.

And you use the toDateTime(timeZone) to return a DateTime object but applying the specified timeZone to it.

So, you can use toDateTime(timeZone).withZone(secondTimeZone) and you will get a copy of the DateTime generated by the first statement (toDateTime(timeZone)) but, with a different time zone, perseving the milisecond instant. And if you use toDateTime() with no parameters, will only retrieve a DateTime object.