I can reproduce that issue on Meta. I’m using the latest version of Chrome on Ubuntu. It took a few tries to reproduce it. What seemed to trigger the issue when I opened the topic timer on this topic was that I selected the “Custom date and time” option and then closed the topic timer modal. After doing that, I’m consistently getting the “Last” entry with the text “Invalid date” being displayed when I open the topic timer modal on Meta.
I’m not seeing any errors in the console. There is a deprecation warning that may be related though: value provided is not in a recognized RFC2822 or ISO format. moment construction falls back to js Date(), which is not reliable across all browsers and versions. Non RFC2822/ISO date formats are discouraged.