One of the most important questions to ask yourself if you’re giving someone a lot of leeway is this: would you cancel on them for the same reason?
Being reliable is a really great strength, but it’s not one that many have.
Flaky people often want the option to A) see if anything better comes up or B) be able to do whatever they feel like the day of. So they’ll ask if they can let me know whether or not they can do something the day of the plan, which leaves me hanging in terms of planning my life.
Be brutally honest with yourself about how long things take. Sure, under the best of circumstances, a certain drive takes 20 minutes. But how often do the best of circumstances occur? There is almost always a traffic jam on that route and then finding parking takes another 15 minutes. In your calendar, leave yourself plenty of time to get places.