ARGH! My apologies- hope I catch you before you go off on a wild goose
chase. Use this formula instead:
=OFFSET($A$1,ROW()-1,0,1,1)+OFFSET($A$1,ROW(),0,1,1)

For this formula to work, it must start on the same row that your
"present data" range starts. Where the formula says "$A$1", change the
$A to the column that holds your present data range (include the $
anchor). Also, the formula assumes recognizes the entries as dates and
times (not text, for instance); format the formula cells as date/
time.

Dave O