How do i turn a column of these (62000) into this (06:20:00)

Hi Guys

I have a problem. I've been trying to make an add-in that compares two columns and highlights any times that are more than 15 minutes apart.

The only problem, is that one of the columns is in hh:mm:ss format and the other is just numerical.

I've attached the file if anyone wants a closer look.

I'm eally stuck and would appreciate any help

Thanks

Colin