Adding time to multiple projects or tasks

Is it possible to add time for multiple projects or tasks during the same timespan?

No, it is not possible to add time to multiple projects or tasks at the same time. Time can only be associated to one project or another, and the same is true about tasks. For example, If you tracked time on the timer to one project and then manually try to add new time on the site for the same timespan, but for a different project, it will overwrite what you tracked and that time would now show only for the new project you selected.