Hi,
Welcome to our community! 
First of all, good job on analysing the problem. If the output in the console shows the expected values but nothing happens in Unity, it might be that the UI Image is not connected with the Timer component. Have you already checked that? Maybe there is a second Timer, for example, in your Assets folder. Things in the Assets folder are not in the scene and cannot be seen by the player.
See also:
- Forum User Guides : How to mark a topic as solved