Update Timestamp in accordance with new rules

Timestamp rules for notifications:

- we should go with relative within the first two days and then absolute:
- Just now* (for the first minute or two)
- x minutes ago* (for the first 59 min)
- x hours ago* (for the remainder of the calendar day)
- Yesterday* (for the previous calendar day) with the addition of a *timestamp* for anything
within the previous calendar day.
- *absolute day and date* for anything older than that.

The issue specifies notification timestamp rules but names no implementation files or tests. Locate the notification timestamp entry point and existing timestamp tests first, then verify the relative and absolute formats across the stated time boundaries. Done means each listed rule is represented and covered by tests.

Written by the indexing model from the issue text.