In addition to the recommendations already made, I would recommend using the proper tool for the job. Just create a reminder in what ever calendar app you may use or personal assistant or the like. When ever I give the dogs their flea and worm treatments I say “Hey google, remind me to apply flea treatment in 90 days” or “hey google, remind my to give worm treatment in 30 days.” Alexa has similar capabilities as do most calendars I’ve used (Google Calendar, Outlook, NextCloud), just without the voice commands.
openHAB is pretty much capable of doing anything, but that doesn’t mean it’s good at everything. When it comes to sending a reminder like this, there are likely other tools that are much better suited to the task.
See Design Pattern: DRY, How Not to Repeat Yourself in Rules DSL.