Time intervals (like days) -- use iCalendar binding, DateTime items or smth else?

Setup, Configuration and Use Items & Sitemaps
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see DateTime List Item, DateTime Standalone Widget. Also look for other useful stuff in the Marketplace. Rule templates, UI widgets and more can be installed and used from the marketplace through MainUI.

Also look at the rule templates. Alarm Clock Rule could be used here too run a rule at the defined date time to change your vacation mode item.

So you’ll get both the UI widgets and rule for free, little to no oding required.