Wrong timezone used in Overview graph

It's apparent that the graph showing incoming/outgoing message number by date, must be using UTC, despite my having set the time zone in my settings (which works with all other displayed dates in the UI).

Start with the Overview graph that shows incoming and outgoing message counts by date, then compare its timezone handling with the UI dates that already respect the user's settings. Confirm the graph uses the configured timezone rather than UTC, and verify the displayed counts and dates against a non-UTC setting.

Written by the indexing model from the issue text.