Thanks for your message. The issue may just be related to cache, actually to the PHP Session that caches most configuration settings. In case you have recently changed the date format, it may be possible that the datepicker calendars are still using a different format, and that would explain why you are getting that error message. It should be sufficient to test the form on a private window with your browser where no cached settings will be applied.
However, we just opened the link of your website home page, and we noticed that the datepicker calendars are not actually the default ones of VikRentCar (jQuery UI Datepicker which natively ships with WordPress), but they are rather being controlled by the “bootstrap datepicker” library.
This would indicate a conflict with a third-party plugin or with some default settings of your Theme. Our suggestion in this case is to turn off any possible conflicting feature with the Theme or with third-party plugins so that only VikRentCar will handle the loading of the datepicker calendars that will use the proper date format.
Lastly, our suggestion is to only use Widgets (blocks) in the home page, and to use Shortcodes on any other page of your website except the home page. This is because the home page has got no permalink, and so the URL routing functions during the booking process may not work as expected. Instead, widgets like the “Search Form” will let you choose the landing page URL from which the booking process will start (i.e. /book-now).
We hope this helps!
The VikWP Team