The timeline macro groups tiddlers by the day found in a date field. The Recent tab in the sidebar is a single call to it, which is all the Output below is.

The nesting happens inside the macro:

• the outer filter uses eachday to yield one tiddler per distinct day
• the inner filter uses sameday to collect every tiddler falling on that day
• dateField chooses which field to read, so the same macro groups by created just as well as by modified

The five dummy payload tiddlers below carry hand set modified dates. Nothing else here has a date field, so they are exactly what the timeline finds.

Pass format to change how each day is headed, and limit to cap how many tiddlers are considered.