When it comes to productivity, time-blocking your schedule is a valuable tool. But the No. 1 thing that people do wrong is confuse their calendar with their to-do list. A to-do list is a place for your tasks to sit until you cross them off. A calendar is a place for non-negotiable time commitments.
Your tasks are negotiable time commitments. You might plan to do a task on Tuesday at 8 a.m., but if something comes up, it gets bumped. Maybe a meeting ran long, or you arrived at the office later than expected. Not having a chance to work on that task just means that you need to schedule some other time to get it done. It's negotiable.
But, you can't plan to deliver a live presentation on stage at a conference and then not show up. You can't plan a customer demo and skip it last minute. You can't plan a board meeting, or a flight, or a networking event, and then push them to later in the day. If it's not negotiable, it belongs on your calendar.