You want CSS and JS to be cached. It speeds up the loading of the web page when they come back. Adding a timestamp, your user's will be forced to download it time and time again.
If you want to make sure they always have a new version, than have your build system add a build number to the end of the file instead of a timestamp.
If you have issues with it just in dev, make sure to set up your browsers to not cache files or set headers on your dev pages to not cache.
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