What generates the In/Out CellLabels in Mathematica and how can I add automatic Timing to them?

Question

When Mathematica evaluates a cell, it gives the Input cell and Output cell the CellLabels In[$Line]:= and Out[$Line]= where $Line is a counter that get

Answer 1

Another way of doing this would be to set EvaluationCompletionAction -> "ShowTiming" which will display timing information in the status bar of the notebook window after each evaluation.