A checkbox is a thing to toggle off/on to select a state. As I understand your description of the app it toggles itself off and the user toggles it on. That's

A checkbox is a thing to toggle off/on to select a state. As I understand your description of the app it toggles itself off and the user toggles it on. That's not a checkbox but a button. If the user can toggle it off that's more like an undo-operation, still not a checkbox. Of course in the sense of "checked as done for the day" it is a "check"box. But it is a little unexpected without contex when reading about a "checkbox".