Hello.
1) For hiding clock icons, you can make this fix.Please open this file https://www.institutogomez-ulla.es/wp-content/plugins/booking/css/client.css
( you can check how to edit files in WordPress menu in this article https://wpbookingcalendar.com/faq/how-edit-file-in-wp-menu/ )
then add this code to the bottom of this file:
div.block_hints .block_time,
div.datepick-inline .timespartly {
background-image: none;
}
Please do not forget to clear browser cache before testing these changes.
2) Relative issue with hiding days. Probably, its because of conflict with some other plugin or actual theme.
Please try to deactivate one by one all your active plugins at the WordPress > Plugins menu and make testing after each plugin deactivation.
If its not help, please deactivate all your plugins and active the default WordPress theme. And then retest it again.
We just need to find the reason of this issue.