Can somebody tell me if the last calculation time of a project planning and the date used for the calculation is stored somewhere in the mySQL table? If yes in which one?

Table planningelement

Other question, I have noticed that each calculation the table assignment is recalculated, is there an history of the previous assignment calculation ?

No, not on assignment, even if yoju can view history of activity that will show changes coming from changes on assignments.
But you can store a baseline of project, that will store all data of the planning (planningelement, plannedwork) to be able to display it on the gantt chart.

But I've been looking in the table planningelement and I cannot see really the date choose for calculation unless if on the project there is no work done, the plannedstartdate will be updated and accordingly plannedenddate.

Thanks for the tip about Baseline.

The date used to start calculation is not stored.