The A-Z or Z-A buttons only sort the currently selected column(s), using the
first one as sort order. In your case, if you selected the column with the
length of assignment, it got sorted and the formulas where screwed up , I
suppose.

By the way, should not your formula be Length: = B1-A1, as you should use
cells on the same row?

In your case, select the whole set of data to be sorted (or only one cell if
your range has no blank rows or columns), and use the menu option Data
Sort, then select the proper field(s) -- up to three -- for sorting.

Stephane.