How to Compare Two dates with Previous/Old Two dates

Question

I've to compare Start Date and End Date with the previous entry of Date duration in database, Actually i want to perform advance salary, I calculated No of Months between start date and end date and created the salary for calculated months in advance, now i want to make it sure that if i create the salary and months lies inside the advance salary created then show a validation error msg that Advance Salary Paid for this duration. 

 

Thanks in advance.