This is because you are booking within the hour of your walk. Some of our computers may be off a minute or maybe even as much as 5 min. So for example, if our computer reads 5pm, but your computer reads 4:59am, and you think you still have the minute to sneak in the change, the system will not allow you to as it goes by our computer time. To play it safe it’s always best to reschedule as soon as you know. Otherwise, you can play it safe by rescheduling 65-70 min prior to your walk.