Interactively...
Choose Cell ▶ Notebook History to get an interactive plot of when the cells in a notebook were edited. Hover and click the plot for further information; see Notebook History Dialog for complete details:
Programmatically...
The times when a cell was edited are recorded in its CellChangeTimes option. Extract a notebook’s cell expressions along with their change times with Cells and NotebookRead:
Extract just the cell change times:
