Computing turnaround time down to minutes/seconds

I'm trying to compute the turnaround time of my transactions using networkdays. I need to show the result in a h:mm:ss time format.
So far, I have this formula: =NETWORKDAYS(G2,H2,Holidays)-1-MOD(G2,1)+MOD(H2,1)
But it can't capture some items that are only a few minutes/seconds apart.

Ex.
Start date: 4/6/2014 19:09
End date: 4/6/2014 19:13

With my current formula, the result shows only #########

It seems for work on other entries but not for the dates like the example above & I can't figure out why that is. Any ideas on how to fix this?

Thanks!