Using your specific example, of wanting the time changed to be input into the cell directly to the left of the change being made.
The following code should work.
Right click on the sheet tab that you want this to apply to (sheet1, sheet2 etc.)
select the view code option
paste the following into the code sheet that appears
Private Sub Worksheet_Change(ByVal Target As Range)
Static runme As Boolean
If runme = False Then
runme = True
Cells(Target.Row, Target.Column - 1).Value = Now()
runme = False
beware that this will run wherever you change a cell. You could modify it so that it only ran if the change was made in column B for example by changing the if to read
If runme = False and target.column=2 Then
Hope it helps