Andrew,
1) Format cell A1 to Time (13:30) in the menu.
2) Format cell B1 to Custom (hh:mm;@) in the menu.
3) In cell A2, type"=SUM(A1+$B$1)
     This will add the start time to whatever value you have in cell B2,
right now you want 5 minutes.
4) Copy and Paste A2 down the column until you have enough data.
5) If you decide to change the interval, all you ahve to do is change the
time interval in cell B2.