First, set up a working day of 7.5 hours.
Whenever I try to create a task taking 10 days (for example), I end up with a
10d 5h task. If I switch to Gantt Chart and try to modify the work
(double-click/Enter in the 'Work' field), but don't modify and press 'Enter'
again, the field would "grow". Subsequent 'enter's would only increase this value.
Also, in the exported HTML file, the 10 days task (which appeared in the Gantt
Chart as having 10 days and 5 hours) is only taking 9d 3h.

NOTE: The 7.5 hours work day is not unusual. I work for a canadian company that
has a work day of 7.5 hours. I also worked before for a french company. They
also had a reduced work day. When I left the french company, their government
was thinking about having a 7.25 hours work day!! :)
I can't just use an 8 hours work day. Each month we generate a report saying how
many hours we spent on each project.
Planner seems to take the 8h work day for granted! But it's not...