With display data recording, if I set autosave in units of 1 day, from what data to what data constitutes "1 day"?

It's one day from the moment you press the START button. Note that if you're also using the memory timeup function, you can start a new file at, for example, 00 hours and record 1 day's worth thereafter. In this case files are created daily at 00 hours, and recorded for the specified time.