Excel's Inability to follow a pattern of adding 15 sec

[Demni009]

Anyone know why excel would shift increments on me?

Basically I type a date and a time in a cell:

3/29/18 10:25:08

Then I type a date and time 15 seconds later in the cell bellow

3/29/18 10:25:23

I highlight both cells and hit the little black square to fill the column in till the end of my data set. Part way through I get a single cell with a 16 second increment then it goes back to doing 15 second increments. This happened enough times to shift my endpoint time by five minutes. Why does this happen?

 

[mole999]

could you add it this way from your reference =B3+TIME(0,0,15) or are you adding

0.000173611111111111000000000000

<colgroup><col width="264"></colgroup><tbody>
</tbody>

 

[SpillerBD]

I think you have a culmination of rounding errors using the fill down method.
The 24 hour/60min/60sec fractions, and then don't forget base10 /base2 conversion and back again to display base10 (and the other restrictions of 80bit binary math)

Mole's suggestion should be helpful, but at these fine precision levels don't be surprised if you still see some errors.