I did the following with Excel 2003 and it seems to work.
One-time-setup of the spreadsheet
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Start with
File > New > Blank workbook
To allow circular references, use
Tools > Options > Calculation
Check the "Iterations" checkbox.
Set "Maximum iterations" to 1 (one).
Type the letter x into cell A2.
In cell A1, put
=IF(A2="",A1+1,-1)
In cell B1, put
=IF($A$2="",IF(ROW()=$A$1,NOW(),B1),0)
Extend this down for at least as many rows as there are steps for the
time readings.
Select column B and use
Format > Cells > Number > Custom
with the pattern
hh:mm:ss.00;;
In columns C onward, put calculations using time readings that will
appear in column B later. For example, putting
=B2-B1
in C2 and extending down would calculate difference times between steps.
Save the workbook. It'll serve as a starting point each time.
To use the stopwatch
--------------------
Open the saved workbook.
To activate the stopwatch, delete the contents of A2.
To click the stopwatch, press the F9 key . A new time reading appears in
column B each time F9 is pressed.
Once you are finished, you can (optionally) reset the spreadsheet by
typing x in A2 again. This way, the spreadsheet can be re-used without
opening it again.
Caution
-------
Allowing circular references can be a risk. Later, if you make a
circular reference in the workbook by mistake, Excel won't flag it as an
error.