welcome to the Kibana community.
If you've found any bug or issue in the date picker I'd recommend to fill an issue on the EUI github repository.
As for the specific question, in the hypothesis you are testing a fix for kibana, the right folder to apply the fix is the lib folder rather than the es. Note that you'll lose everything in there on the first reinstall, so perhaps an fix/enhancement request issue on the EUI repo would be best.