Know which question you are asking
A date names a day on a calendar. A timestamp identifies a moment. “How many dates apart?” is different from “How many hours have passed?” AgeUse answers the first kind of question and does not accept a time of day.
Keep daylight-saving changes out of date counts
The tools compare date-only values on a consistent calendar basis. A local clock change does not add or remove a date from the count. This is why the result should not be treated as an exact number of elapsed hours.
Check the reference date
The initial reference date comes from your device's local calendar. Someone in another time zone may already be on the next date. If you need to compare results with another person, choose the same explicit reference date in both tools.