A timer that counts the time until... actually nobody knows what this counts. But you should be able to download the thing and run it on your own web server and import it as a browser source in OBS or Xsplit or whatever.
If you don't want to host it yourself you're welcome to use the one I'm hosting at https://astrocatfreitag.com/code/stream-countdown-timer/countdown.html. If you download it from me tho you're stuck with my settings that work for my stream. Don't ask me to change it for your stream. You have the source available change it yourself. My version is also subject to random changes at any time or just not working for testing.
Add as a browser source to OBS
look it up if you don't know how to do that
paste in my URL above or your own URL if you're hosting it yourself
as written this fits into OBS in an 850x245 box. FPS should be fine at 30.
erase the custom CSS because I wrote some custom CSS or write your own CSS
the custom css will hide the text
this is bad
you can also just download the files and edit to your liking and add the HTML file as a local file in the OBS browser source options.
as currently written, the countdown will fit in an 868x288 size box in OBS Chrome says the source is actually 864x326, this is the size I use