XL chops off the number after 15 digits and puts in zeros (placeholders).
You can get XL to display all of the digits by formatting the cells as text,
but that won't help accuracy if you perform arithmetic operations.
You may need to use something other than XL. You might also look around for
any XL add-ins that might help overcome this limitation (I think I've seen
one before, but have no clue where to find it now).
You could look at using VBA since it has data types that can accomodate more
digits. Your data would need to be stored in the cell as text. Suppose A1
is '1.2345678901234567890, then Test(A1, "*", 2) returns 2.469135780246913578
I don't know of a way to get XL to display times to a greater degree of
accuracy than hh:mm:ss.00
Function Test(strInput1 As String, _
strOperation As String, strInput2 As String) As String
Dim varInput1 As Variant
Dim varInput2 As Variant
varInput1 = CDec(strInput1)
varInput2 = CDec(strInput2)
Select Case strOperation
Test = CStr(varInput1 * varInput2)
Test = CStr(varInput1 / varInput2)
Test = CStr(varInput1 + varInput2)
Test = CStr(varInput1 - varInput2)