In the beginning of my career helping PC users, I noticed some folks using Lotus 1-2-3 to write letters and I noticed them using MS Word and MS Excel to maintain lists. In both of these cases, they "got away with it", the software allowed them to trick it. But in the end, using the wrong tool will bite you. FWIW, the proper tool for letter-writing is a Word Processing program and for a list the proper tool is a File or Data-Base Management program.
It is NOT always obvious where to record a task or an event, in the Date Book or the To Do List. The problem will be clearer when I describe how to choose one or the other.
A Date Book entry is an event ("when"). Examples are: a date with Ellen, not to be confused with a date with Jane or Tracy. You can't just interchange them. If you try, they will show you how interchangeable they are. A doctor's appointment, or a talk with your lawyer at his place are events, therefore Date Book entries. Hesitate before making "go to work" into a repeating Date Book entry. If you work a "normal" work week -- Monday thru Friday, from 9:00 to 5:00 -- you will just be cluttering the look of the day. Hesitate before coding "go to the movies" as a Date Book entry; at best it is a To Do List task, as you will fit it in when you have time. And a To Do List entry is typically a task ("what").
Here are the "rules."
The easy way to look at the difference is: events are Date Book entries, tasks are To Do List entries.
Where they become fuzzy is a) when an event has flexibility in its timing (like a movie that opened in your neighborhood and will play for a week or two) or b) when a task must, for any reason, be performed by a set date and that date is imminent (clean your apt before your date with Jane).
So, the simplest way to state the rule is: specificity demands the Date Book, flexibility permits the To Do List, and a To Do List entry with a strict Due Date is the way you combine both notions.