Hi Yves-Luc 

Thanks for reaching out! It is not possible to include negative dates when using the Timeline Shape library. As a workaround I recommend creating a timeline from scratch using basic flowchart shapes and text boxes. You can then manually add enter the BC date. 

If this is a feature you would like to see implemented please send it to our development team with our feature request form.

I hope this helps! Please let me know if you have any further questions.