Car rental wordpress system Date format modification - repost

Hello freelancers. I'm building a website ( [login to view URL] ) and i need a date format modification when a user pick up dates to book. Right now the system only works with "n/j/Y" format and i want to change it to d/m/Y or dd/mm/yyyy. When you trying to change it the system doesn't work and no results are shown. In php files there is a regex date pattern that validates the date format and i don't konw how to make it work. I will supply you with full access on the files. We really need to end it, asap and we need a serious php expert for the job. Thank you!