ISSUE SUMMARY

In previous versions of MongoDB, the date extraction operations $year, $month, etc. could only extract the information from Date types in the UTC time zone. This meant that
it was not possible to do grouping by date/time information in a local time zone. MongoDB does not have any concept about a server-wide local time zone.

In versions 3.5.11 and newer, MongoDB still does not have a concept of a server-wide time zone, but the date extraction operators ($year, $month, $day, $hour, $minute, $second, $millisecond, $isoYear, $isoWeekYear and {{$isoDayOfWeek} allow for a secondary format to specify a time zone to express a local time.

USER IMPACT

This is not a BC breaking change. The original syntax of the operators is still functional:

> db.c.aggregate({hour: {$hour: '$lastUpdatedDate'}})

A new syntax is now also supported, to allow for the extra timezone field:

> db.c.aggregate({hour: {$hour: {date: '$lastUpdatedDate'}}})

> db.c.aggregate({hour: {$hour: {date: '$lastUpdatedDate', timezone: 'Europe/Amsterdam'}}})

TECHNICAL DETAILS

The implementation of time zone support is done through "timelib". Several formats for the timezone field are allowed:

• An "Olson Time Zone Idenfitier": America/New_York, Europe/London, etc.
• A string describing an ISO 8601 time zone specification: -05:00, +0445, -03, etc.
• The strings Z, UTC and GMT
• Or an expression that evaluates from one of the above. This allows you to fetch the time zone information from a separate field in your document.

ADDITIONAL CHANGES

The $dateToString operator has also been updated (SERVER-28610), and the new operators $dateFromParts and $dateToParts (SERVER-28613), and $dateFromString (SERVER-29208, SERVER-29209) have been added, each also supporting time zones.