DateDiff is an extension to MediaWiki that provides parser function #dates allowing to return a list of intermediary days.
See also the "CHANGELOG.md" file provided with the code.
(1) Obtain the code from GitHub
(2) Extract the files in a directory called DateDiff in your extensions/ folder.
(3) Add the following code at the bottom of your "LocalSettings.php" file:
wfLoadExtension( 'DateDiff' );
(4) Go to "Special:Version" on your wiki to verify that the extension is successfully installed.
(5) Done.