To frame it another way, your dentist has agreed to a particular triggering condition. (Whether they realize that's how their country works or not.)

We can all make pretty-good guesses about when that condition will be fulfilled in N seconds, but until it actually finally happens, the true occasion could land somewhere else.

"Three months later at 2PM where I live" is not so different than "when the thrush knocks during the setting light of Durin's Day." You can guess that it's N seconds from now, but you might be wrong.