A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
Hi finnapx,
When adding a Countdown Timer web part in SharePoint Online, we can only set the Date and Time manually.
Considering you’re using Flow to achieve your goals step by step, as you said, you may try to trigger the timer with a Flow. As our forum focus on how to deal with the out-of-box issue of Office 365, we are really sorry for couldn't provide a better suggestion for you. We sincerely recommend you raise this in Flow community forum, which is the official forum for Flow. Experts there would conduct in-depth research on the problem you met and can provide you with more professional help.
Highly appreciate your understanding.
Best regards,
Madoc