You are correct, there are only 609 intervals.

Sandy Mann wrote:

Bob,

I think that you should be dividing by 609 not 610. With 610 the intervals
are 1:15:09 and cell A609 has a value of 5/8/2008 10:33:42 and the last
interval to A10 is 2:30:18.

Dividing by 609 results in all intervals including between A609 & A610 of
1:15:17