The 3600 you want to display is not a time...but, perhaps a custom number format can help.

Select the cell
<format><cells><number tab>
Category: Custom
Type: General" sec"
Click [OK]

Now, 3600 will display as: 3600 sec

Next...To Excel, time is calculated as a percentage of a day.
Noon is 0.5 (half a day)

Your formula will need to convert the duration (in seconds) to a percentage of a day.

Try this to calculate StopTime:
=+A1+B1/(60*60*24)

(60 sec/min * 60 min/hr * 24 hrs/day)


Is that something you can work with?