Working at a callcenter, we did time math all the time. Your best option is to leave everything in seconds for calculations, changing only to minutes when you output for humans to read.

>It comes from (date_out - to date_out) / 60
>
>Is it a wrong way ?
>
>I am running the software with no problem, but now that you spotted the problem, I thing its worth to investigate
>to avoid future problems