Calendars, on the other hand, cover the entire spectrum of time. Past, present and future. They are the closest thing we have to a time machine. Calendars allow us to travel forward in time and see the future. More importantly, they allow us to change the future.

Treating todos as calendar events is helpful because calendars introduce constraints. A calendar forces you to estimate how long each task will take and then find empty space for it on a 24 hours × 7 days grid, which is already cluttered with other things. It’s like playing Tetris with blocks of time.