have a function but if the answer is a negative I want it to say zero "0"

I have an install date and date of my monthly report, I need to know how many days have lapsed at the time of my report from that install date, some of the entries on my sheet have not been installed yet so they are showing up as a negative number and I need them to show as a zero "0". Also if someone has completed their time during the reporting period, I need the days to stop counting at their removal date. This is a constant sheet that I will be adding too daily and no one will be taken off, so I will be doing a lot of sorting to make my reports to the individual agencies. I am attaching a copy of my worksheet. I appreciate any and all help I can get.

I am not "tech" savvy! Please be easy on me with the jargon! lol

Thank you!

Laurie

PS: if you can explain what the (Tags thing below is for?