You need groceries. You also have a thing at 4. Your brain says "I have plenty of time" and then you spend 20 minutes looking for your list, 15 minutes in the cereal aisle, and another 10 in the parking lot checking your phone. A backward plan from your next commitment tells you when to leave, when to shop, and when to head back.
Time blindness hits hardest on unstructured tasks. A grocery run has no fixed steps and no external cues. You wander, you browse, you forget what you came for, you remember something else. Then checkout takes longer than expected. Then you realize you need gas. The quick errand expands to fill whatever time was available, and then some.
Ready Time works backward from when you need to be back (or at your next commitment). It tells you when to leave, gives you a fixed window for shopping, and alerts you when it's time to head to checkout. You still shop however you want, but the timer keeps the errand from eating your whole afternoon.