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

It's possible to add evaluation timestamps to Mathematica cell labels by selecting "AddTimeStamp" in the option inspector settings for EvaluationCompletionAction. Moreover, a list of options can be given by editing the entry, so using {"ShowTiming","AddTimeStamp"} I get both an evaluation duration in the status bar and input and output timestamps in the cell labels.