Hi Thomas,
this is currently not possible. Diarium is built around date-based journal entries, so every entry belongs to a specific day.
This feature has already been requested here:
For now, the closest workaround is to create an entry on a chosen date, use the heading as the note title, and add a tag such as “Notes” or “Insights” so those entries can be found again later.
Please continue the discussion in the existing thread, so votes and feedback stay in one place.