Here's perfect example why Matlab excels over Excel... :)
One doesn't need to keep all the intermediary variables unless there's other need for them; kept here so can look at them and see how matches up.
The text file and the Excel file aren't the same data and the Excel file has the European "," for the decimal point so it's hard to read here in the US w/o other machinations so couldn't directly illustrate it does give identical result...
BTW, despite the Q? title, there's only a first derivative, not second...if need the second, you just "rinse and repeat" the above or use the second, optional argument to diff to compute it directly if don't need the first but this was just a step along the way...