• Because you're more absorbed by whatever catches your attention in the present moment, having ADHD means you're not as good at putting yourself "outside" of time and figuring out how long things will take.
• ADHD isn't a simple "attention deficit" as much as an inability to regulate attention. When you have ADHD, it can be hard to pay attention to things, but it can also be hard to remember to stop paying attention to things once you're engaged. This "hyperfocus" can lead you to stay focused on an activity even when you should be moving on.
• Being caught up in your impulses in the here and now makes it easy to lose track of time.
• When planning an activity, having ADHD makes you prone to thinking in general terms and skipping over the details. If you don't consider the fine points of exactly what's involved in doing something, there's a good chance you're going to underestimate how much time you need to do it.
• People with ADHD are often procrastinators who don't get started on things until there's a sense of urgency. And if you start something late, there's a good chance you're going to finish it late, which throws off your schedule for everything you have to do next
• Having ADHD tends to make you impatient and very averse to boredom. As a result, you don't like waiting and you aren't a fan of getting places early, so you might try to arrive to events exactly on time, with the predictable consequence that you actually end up just being late.
• Since you aren't a natural at planning ahead, you're not likely to think about things you have to do until they're really pressing. When going somewhere, you might not start getting ready to leave until there's a danger of being late.

So if you have ADHD, your focus on whatever captures your attention in the present, your knack for procrastination and your frequent failure to plan ahead conspire to make you late to things. The good news is that once you're aware of these tendencies, you can take steps to counteract them. Anything that "outsources" your ability to plan ahead is good - alarms, schedules, etc. These tools can act as reminders when it's time to move on to the next thing.

If you find yourself asking, "Why am I always late?" try using the above list of ways ADHD symptoms can prevent you from being on time as a starting point for figuring out how the disorder is causing you to get behind schedule. Even if you have a lifetime of chronic lateness behind you, you can get to the root causes of your tardiness and take steps towards becoming more punctual - better late than never!