Website Database links need Fixed

We are a small agency, based in Edinburgh, providing on-site chair massage, to companies and their employees across the whole of the UK. We are looking for assistance in fixing an issue which has recently occurred with our websites. Our websites are windows based and have an internal calendar, and customer booking functionality which is managed via an Access dBase. The code for the calendar/booking functions are written in in C#. The functionality was custom made for requirements After many years of successfully making bookings and recording the data on the Access dBase, we are now encountering an issue with the data being recorded on the Access dBase. The application (front end) allows customers to book appointments and the calendar function displays the dates in UK format (dd/mm/yyyy). Upon making a booking the application issues an email to the customer confirming the booking. The date recorded on the email issued to the customer is in date format dd/mm/yyyy. We are now finding the record created on the Access dBase is being converted to the US Date format (mm/dd/yyyy), this is causing us major issue when issuing reminders to customers, as their booking is now incorrect. i.e. a booking for the 05/06/2019 (5th June 2019) is now being recorded on the database as 06/05/2019 (6th May 2019) The issue affects 2 websites so any fix will have to be applied to both sites. We are not looking to re-develop the websites just have this issue resolved.