Some of the most basic tools used by project managers are important because they are the ones that keep you and your team always on time. Schedule-keeping is critical for maintaining a project's progress, presenting said progress to superiors, and keeping on top of any critical work that needs to be done at a specific time. For that reason there are many tools that are widely available in order to make and keep appointments in written form as a reminder.
Analog calendars are the traditional pen-and-paper calenders that we've all seen before. They are still fantastic tools in an age of e-mail and software because they can be placed in direct view where you can see them and constantly be reminded of any upcoming appointments. There are downsides to this type of calendar. You have a limited amount of space, and if you mark down the wrong date it's difficult to change what you wrote, especially if you used ink. These calendars need to be updated every year, which can be bothersome to a degree.
If you find that you run out of space on calendars too quickly, or you just want a quick reference tool to have on your person to keep your schedule, day planners make excellent tools for the job. Day planners allow you to schedule things by the hour of each day, sometimes even down to 15-minute intervals Usually, day planners include a variety of additional features such as a place for contact information to be stored and even a yearly calendar.
There is a lot of software out there that can mimic analog calendars and even surpass them in other ways. Calendar software, like Google Desktop and Mozilla Thunderbird (with Lightning 1.0b2 add-on), allow you to have an unlimited amount of space to note appointments. They are able to break down each day of the calendar into minutes to keep a more organized schedule, if you so choose.