This formula worked for me:
=OFFSET($A$1,(ROW()*2)-2,0,1,1)+OFFSET($A$1,(ROW()*2)-1,0,1,1)

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

Dave O