Funny coincidence -- I just received an email with a countdown like that today and was very curious. In my case, the email was using: https://www.realtime.email/
Now you can do it yourself haha ;) I still have to cleanup the code a bit. Thinking of creating a class which allows you to create such image by just writing a multidimensional array, so less code to write. But I couldn't wait to share this already so... But updates are coming soon.