OP uses their calendar as a supplement for their the todo file. There is a lot of functionality implied in that decision:

- calendars have mobile apps which enable quick and precise entry

- calendars understand time spans

- calendars have many options to display events

- calendars have cloud syncing

- calendars are backed by a queryable data store

Not saying using a text file is good or bad, but I think a more accurate title would be “my productivity app is a never ending txt file and a calendar app”