You're welcome to ask any questions about GanttProject here, but we strongly recommend to read the FAQ and search before asking. This may really help you to get the answer faster.
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Many questions were asked and answered previously on our old forum site and in the issue tracker. You can use our Google Custom Search to search all relevant pages.
Frequently Asked Questions
How to change week numbering?
GanttProject uses calendar settings corresponding to the chosen interface language. Switching interface language to e.g. French will start Week 1 in 2016 on Jan 4. If you're using English interface language then you can choose between English US (it is just "English" in GanttProject settings) and start Week 1 on December 28 (correct for US) and English (United Kingdom) which follows European standards.
Where can I find a user guide or manual?
We have no user guides or manuals, sorry. Instead, we can offer you an application which is so easy to use that needs no manuals, provided that you don't need guidance in project scheduling per se. If you still need to see how to do the basic actions, consider watching one of the awesome video tutorials contributed by volunteers.
Some of the tricky bits and rough corners are documented at http://docs.ganttproject.biz
Is it possible to use hours as task duration?
Can GanttProject do resource leveling?
I can't change the dates of my tasks!
Usually it means that either there is a dependency between your tasks and GanttProject is just doing its job and is trying to make your constraints happy. Please read the article "GanttProject Scheduler Explained" which may help you to understand what exactly forbids dates change.
If you don't find the answer in the article and believe that there is nothing in your data which prevents tasks from being moved, please submit a new ticket to the issue tracker, supply a reproducible scenario and attach an example project file.