This seems to work with Excel 2003.
First, permit circular references by checking the box at:
Tools > Options > Calculation > Iteration
Start with all cells empty.
In B1 put=20
and copy down if needed. Format column B as desired; as a date, for example=
Next enter a value into column A next to one of the formulas.
The date (and time, if included in the format) should appear in column B ne=
xt to the value. What's in column B does not change if the file is saved an=
d re-opened in the future. It also does not change if somebody overwrites a=
value in column A with something different.
If a column A value is deleted, the corresponding column B cell should beco=
me empty too.=20
Caution: permitting circular references can be risky and is discouraged by =
many. If somebody later puts a circular reference in the file by mistake, E=
xcel will not catch the error.