I have the same problem.

I'm not sure if this answers your question, however I'm also using the timesheet modules and create invoices from the hours entered by employees.

By default, we enter working time in number of hours. So each day we enter 8 hours. However when we print timesheets, working times are shown in days. This behaviour can be changed in the Unit of Measurements form by making hours the default UOM for UOM-category Working Times.

Is that still true ? I have the same problem today.

My timesheet is in HOUR and when i invoice it it doesn't convert the hours on days

Example : 8h in timesheet = 8 days on my invoice

Do someone has the solution ?

It is possible to change it in the Settings (> Project Management > Working time unit). However, it will not change any values when you change this setting. It should however use the 2.00 days vs 2:00 hours. It will take it into account when you invoice your timesheets.