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 notebooks cell expressions along with their change times with Cells and NotebookRead:

Extract just the cell change times:

Out[2]=