You think you have an hour to spare, then realise you do not. It is a difference in how your brain perceives the passing of minutes and hours, and there are practical tools that help.
What it feels like
Many people with ADHD describe time as having two modes, now and not now. Something happening in twenty minutes feels exactly as far away as something happening in three weeks, right up until the moment it arrives. An hour of focus can feel like ten minutes, and ten minutes of waiting can feel like an hour. You can lose four hours to a hyperfocus session and have no memory of where they went, and you can also leave a meeting feeling like it lasted forever when the clock says it took twenty minutes. None of this is in your imagination.
1/6 Make time visible, everywhere.
The most useful change is to put an analogue clock with a clear minute hand somewhere you cannot avoid looking at, in your kitchen, on your desk, in the bathroom. A digital clock shows numbers, but an analogue clock lets you see time passing, which is harder to ignore. A big clock works best, even if you don't like how it looks.
2/6 Use a Time Timer.
The Time Timer is a physical disc that visibly counts down a coloured wedge of time. About £30 on Amazon. It exists because the inventor's child had ADHD, and almost every ADHDer who tries one says some version of "oh, this is what people without ADHD see when they look at a clock". Set it for 25 minutes when you start a task. Watching the red shrink does something no app notification can.
3/6 Calendar everything, and double the buffer.
Put every appointment, every task, every social obligation into a single calendar app, and add a buffer that feels comically generous, then double it. If a journey should take 20 minutes, allow 50. If a meeting is at 3pm, block 2:45pm with a "leave now" reminder. The ADHD tax is mostly time tax, and protecting yourself from yourself is the whole game.
4/6 The "what time is it really" check.
Three or four times a day, before you start the next thing, look at a clock and say the time out loud. Say the real time, not the time you think it is. It feels silly, but it helps you notice how much time has passed.
5/6 External countdowns for transitions.
Transitions are where time blindness bites hardest, "I'll just finish this then leave" is the lie your brain tells itself most often. Set a series of alarms before you need to be somewhere, not just one. Fifteen minutes before, five minutes before, one minute before. Label them with what you should be doing at each point. Your future self will thank you.
6/6 Time auditing, for a week only.
Try this for seven days, no longer. Every hour, write down what you did in that hour, in one short sentence. Many people find a big gap between where they thought their day went and what they wrote down. You do not need to keep doing this forever, just long enough to recalibrate your internal clock and notice your real patterns.
A note on therapy and medication
Stimulant medication often improves time perception meaningfully, because dopamine is involved in the brain's internal clock. ADHD coaches and therapists who understand ADHD can help you set up the tools above. If you can only afford one thing, start with the tools above. Coaching mainly helps you keep using them when your motivation dips.