The new functionality adds the timezone argument to the date extraction functions ($year, $month, $day, $hour, $minute, $second, $millisecond, $isoYear, $isoWeekYear, and $isoDayOfWeek). Currently the operators only take a single value:

    { "$month" : ISODateValue }

Because it is not possible to add another argument to this, the syntax has
been extended to:

    { "$month" : { "date" : ISODateValue } }

And then with the time zone argument added, it becomes:

    { "$month" : {
        "date" : ISODateValue,
        "timezone": timeZoneIdentifier
    } }

For example:

    { $hour: {
        date: ISODate("1978-12-22T08:15:00Z"),
        timezone: "Europe/Amsterdam"
    } }

Would return 09 (One hour later than UTC).

The "timezone" argument is optional, and must evaluate to specify an Olson Timezone Identifier such as "Europe/London" or "America/New_York", or, a UTC offset string in the forms: "+03", "-0530", and "+04:45". If you specify a timezone argument it means that the dateString that you provided will be interpreted as it was in that time zone.