Dayfile helps you make notes about your daily life. It works entirely offline, so your notes stay on your device, and it's your responsibility to back them up so you don't lose them if something happens to your phone.
The idea is that making a note should be as low-friction as possible. Open the app, type, close or background it. The note is automatically attached to the current date. You can optionally have predefined categories to help split up the day's notes, but that's it: entries are free text with no attempt at imposing a structure beyond the categories.
There are no reminders or alarms, and no formal support for time tracking more precise than “a day”.
This is almost embarrassingly primitive, which is the whole point. You can use a text editor to make notes, but then you have to worry about having the right file open and entering the date correctly. A calendar app is organised around appointments and arbitrary dates; a journalling app may be more heavyweight than you want.
The emphasis here is simply on making a quick note with minimal ceremony. The less effort it is to make the note, the more likely it is that you'll actually make it.