Letting relief

I have husband and wife clients who purchased a property for £185,000.00 as their main residence in April 2000 and lived in it until August 2004. Since then it has been let. The property will be sold this month for £317,500.00. I calculate that the capital gain = £55,208.33 ( ie the difference between the purchase and sale price x 5/12 ) . Is the calculation based on whole numbers of years, or months, and in any event would the resultant figure be exempt under the rules of letting relief?

Thanks

John.