• config.time_zone = 'Tokyo' # Other than UTC
• User time zone is nil In the case of the above settings, the count when grouped with created_on may not be displayed correctly. !{width: 500px; border: 1px solid #ccc}screenshot.png!

created_on of grouping target issues:

[Thu, 20 Jul 2006 04:04:21 JST +09:00,
 Thu, 20 Jul 2006 04:07:27 JST +09:00,
 Thu, 14 Nov 2019 16:27:41 JST +09:00,
 Tue, 19 Nov 2019 16:27:41 JST +09:00,
 Sun, 24 Nov 2019 16:27:41 JST +09:00,
 Sun, 24 Nov 2019 16:27:41 JST +09:00,
 Sun, 24 Nov 2019 16:27:41 JST +09:00,
 Tue, 26 Nov 2019 16:27:41 JST +09:00,
 Fri, 29 Nov 2019 16:26:41 JST +09:00,
 Fri, 29 Nov 2019 16:26:41 JST +09:00,
 Fri, 29 Nov 2019 16:26:41 JST +09:00]

Expected aggregation results(Date in Tokyo time zone):

> Issue.all.open.group_by{|i| i.created_on.to_date}.map{|k, values| [k, values.count]}.to_h
{Thu, 20 Jul 2006=>2,
 Tue, 19 Nov 2019=>1,
 Tue, 26 Nov 2019=>1,
 Fri, 29 Nov 2019=>3,
 Thu, 14 Nov 2019=>1,
 Sun, 24 Nov 2019=>3}

Actual aggregation results(Date in UTC time zone):

> query.result_count_by_group
{Wed, 19 Jul 2006=>2,
 Tue, 19 Nov 2019=>1,
 Tue, 26 Nov 2019=>1,
 Fri, 29 Nov 2019=>3,
 Thu, 14 Nov 2019=>1,
 Sun, 24 Nov 2019=>3}

# http://www.redmine.org/projects/redmine/repository/entry/trunk/lib/redmine/database.rb#L71
        if postgresql?
          if time_zone
            identifier = ActiveSupport::TimeZone.find_tzinfo(time_zone.name).identifier
            "(#{column}::timestamptz AT TIME ZONE '#{identifier}')::date"
          else
            "#{column}::date"
          end
        else
         # ...

The code above is executed to get the key date for the group.
Instead of returning "#{column}::date" when the user time zone is nil and config.time_zone is set, it should return the date converted to the time zone set in config.time_zone.

This issue is related to #13803.