Looking for container/chassis software

Hi,
I run reefer, dry van, flatbed and dump trailer but I (probably stupidly) decided to add containers to the mix. One of my customers needed a new container hauler and they approached me.

So I've been pulling containers since Dec about 15 to 20 Containers a week. The amount will increase as I can provide more capacity. The problem I'm having is keeping track of what containers are out and when they need to be returned to the port by.

Every container I deliver is just dropped so I have to go back and pick it up. However, I deliver to several locations and trying to make sure everything gets back on time so I don't get charged , is proving to be a little crazy with everything else I'm doing.

So im wondering if anyone knows of any good software that would help keep track of equipment and when it's due back or if someone has any other system they use.