Google Assistant saves a copy of everything you tell it. Here's how to delete everything and turn it off.
Try: A5: =(A3 24) A4
Note time is stored in Google Docs spreadsheets as a fraction between 0 and 1, e.g. 12:00:00 equals 0.5. By multiplying this fraction by 24 you get the time in hours (with minutes and seconds as a decimal fraction of the hour)
Dave Evans: "Designing Your Life" | Talks at Google