Limiting time allowed between 2 date fields

Hi guys. I have two date fields on a Service Catolog request. I want to limit the time between the two fields to 30 days. Here's the real world description:

We loan out equipment for a maximum of 30 days. The form I have created has a start and end date (variable fields). However, people can enter whatever dates they wish. I'd like to limit this to X number of days. Is this simple?

Thank you