Hey @BaBuuuuuu,
I think what you are needing at the moment is more information and foundation in blueprint / widget communication and Game instances rather than the timer itself, as that is the easier part. After that you can implement whichever of the above methods works best for your use case.
Check out these non-Epic affiliated tutorials that will teach you the main concepts of blueprint communication through making a health bar and going over a mini health system using a game instance.
Good luck on creating your counter!