If you are confused about how to install it here is a quick guide.

1. Install "Tampermonkey"
2. Go to the GitHub page for the timer https://gist.github.com/birjolaxew/f342f12836eb8e1958f674efa3fdcd7b
3. Click the "RAW" button on the source code panel
4. Click install
5. Reload https://classic.minecraft.net/ Done

Method 2 (if you had a problem installing)

1. Install "Tampermonkey"
2. Go to the GitHub page for the timer. https://gist.github.com/birjolaxew/f342f12836eb8e1958f674efa3fdcd7b
3. Copy all of the source code from the page
4. Click the Tampermonkey plugin on your chrome top panel (If you can't see it click extensions and then click Tampermonkey.)
5. Click "Create a new script..."
6. Delete everything in the editor.
7. Paste the source code from GitHub.
8. Click "File" then click "Save" (or press Ctrl+S)
9. Reload https://classic.minecraft.net/ Done