Paul Graham offers a nifty essay about two kinds of working schedule.
There are two types of schedule, which I'll call the manager's schedule and the maker's schedule. The manager's schedule is for bosses. It's embodied in the traditional appointment book, with each day cut into one hour intervals. You can block off several hours for a single task if you need to, but by default you change what you're doing every hour.
When you use time that way, it's merely a practical problem to meet with someone. Find an open slot in your schedule, book them, and you're done.
Most powerful people are on the manager's schedule. It's the schedule of command. But there's another way of using time that's common among people who make things, like programmers and writers. They generally prefer to use time in units of half a day at least. You can't write or program well in units of an hour. That's barely enough time to get started.
I'd add a couple of observations from my experience as an interaction designer.
When I was a consultant, I worked in a very small team that got to set its own rhythm of work internally, which meant that we could take the right sized chunks of time to do things most of the time. This had huge benefits for our productivity, both in the volume of work we could do and its quality.
Plus, I have to observe that the calendaring tools that we use — I'm talking, of course, about MS Outlook primarily — are designed (if badly) around the manager's style of schedule, with a simpleminded conception of fixed blocks of time. But almost everyone needs the ability to tell their calendar “every day between 12:00 and 2:00 I need at least half an hour free in which to eat lunch.” And many people need the ability to tell their calendar things like “each week I need 15 hours outside of meetings, including at least two uninterrupted three-hour blocks of time”. This is the kind of stuff top executives have personal assistants to do, but well designed software could do many of the same things. I have a good job, but if someone wants to pay me to design that system, I'm up for talking about it ....