ことになっている = 'the rule/plan says so,' not 'I decided so' — subtle but important

One that trips people up: 「会議は3時に始まることになっている」doesn't mean 'I decided the meeting starts at 3' — it means 'it has been decided/it's the established rule/schedule that the meeting starts at 3,' with the decision-maker deliberately left vague or external (company policy, custom, prior agreement). Compare with ことにする, which IS about a personal decision ('I've decided to...'). If you're describing a rule, custom, schedule, or contract term rather than your own choice, ことになっている is almost always the right pick.