Firstly, I wrote a previous review which gave this 3 stars, but this didn't seem to get published. I've now really used the product and am re-doing that review...
As a test, I set this in Paris, on 21st August 2011, temperature outside was 30 to 33 degrees celsius, with clear skies. I set the timer at about 11:00am and at that time you would call the sun level "blazing". I set the SPF factor to 20. The warning alarm eventually went off at 3:00am on Monday 22nd August (obviously when it was dark). Will I ever use this to protect my child? Not a chance! Its in the bin.