Write about calendars.

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+Calendars and other reminder systems
+====================================
+
+Some things in life have to happen at particular times. For these,
+the calendar is the perfect tool. Any kind of calendar will work,
+as long as you use it diligently for this kind of thing: paper
+or digital, pocket or wall, or even just a text file with dates.
+
+Calendar software has some features that make them particularly useful
+for many people: group calendars, ease of separation between personal
+and work calendars, etc.
+
+You already know how to use a calendar for its basic tasks, so we
+won't go into that.
+
+You can use calendars as an inbox as well. For example, if you need
+to start working on a project in September, but don't need to worry
+about it before that, then adding a reminder about the project on
+September 1 will work fine, but only if you treat the calendar
+as an inbox. If you use a digital calendar, you can have one for
+these inbox items in particular.
+
+Automatic nagging systems
+-------------------------
+
+Another kind of thing is stuff that needs to happen regularly. For
+some of these, digital calendars are still the tool of choice: you 
+could add a bi-monthly reminder to get a haircut to your calendar,
+for example.
+
+Calendar reminders may also be replaced or augmented by cron jobs,
+which run, for example, on the Monday before the second Thursday
+of each month, and tell you to send out an invitation to the
+monthly meeting, which happens on the second Thursday each month.
+Whether you use an actual calendar, or a cron job, is your choice,
+of course: use the tools that suit you best.
+
+Cron jobs have the advantage over calendars that they can be
+conditional: a cron job could, for example, see if you've already
+sent out the invitation, and not remind you if you have.
+
+Some stuff does not fit so easily with calendars. For example,
+suppose you want to cut your nails when they get too long, but you
+keep getting distracted by your computer so that it may take your a
+month to notice that it's time to cut them. And when you do, it's
+because you break a nail, which is a bit painful. After some experimentation
+you decide that twelve days after the previous time is a good time to
+cut your nails. Having your computer remind you about it s makes it
+much more likely that you'll do it when it's time. However, having
+your calendar remind you every twelve days may not work so well,
+because you might be travelling on that 12th day, and the annoying
+flight security theater made it impossible to take your nail cutter
+with you. (This is not a hypothetical example.)
+
+A better solution would remind you twelve days after
+the previous time you actually cut the nails, not after the previous
+reminder. Lars has a program called "nagger" which does exactly that,
+but it is not suitable for others to use (unless you dig editing
+`procmailrc` files, and probably not even then). The nagger remembers
+when you last did something, and after the specified time, it
+starts nagging you every day until you tell it you've done it again.
+