Fix Javascript Calculator

We need a Javascript expert who knows well how to calculate date differences and understand javascript code. • The difference between 2 dates is not calculated correctly. • Strangly the script thinks that Oktober has 30 days and November has 31 days. Same error for all other months. Please see attachment image to understand the error. We also attached the php file that includes the javascript. You can simply test it in your server if you like. we think the job should be done for around 10$ -15$