Compare date field columns to restrict difference in days between the two
The field validation module does what you want, the interface description can be confusing. 


On your Manage fields screen click the validate action link for your relevant date field.
For your field you want to add a Date range2 validation rule.
Name your rule in Rule name to a memorable description.
For column select value2 to validate your end date.
For Cycle of date select global so you can validate against time relative to the start date.
For Maximum date set it to "+30 day" (or any strtotime() compatible relative string) so that the end date is, at most, 30 days from the start date.
Set your your Custom error message.
Click Save rule.