I used the following equation, but when I put it on our palm pilot, the file
is locked. What should I do?

Using circular references and worksheet functions
You can use a circular reference to enter the time when a change is made in
another cell, then maintain that time. Choose Tools/Options/Calculation
(Preferences/Calculation for Macs) and check the Iteration checkbox. Then, if
your target cell is A1 and you want the date/time to appear in B1, enter this
in B1:

=IF(A1="","",IF(B1="",NOW(),B1))
Format B1 as you wish to display date, time, or both. If A1 is initially
blank, B1 will return a null string (""). When a value is entered into A1, B1
will evaluate as "", therefore NOW() will be returned. After that (as long as
A1 remains populated), B1 will evaluate to a date/time and therefore will
return the value in B1 - i.e., the date/time.