Firefox and Explorer won't accept any date format

I’m using Pinboard Theme, and have a single form for people to make a booking – where the date is a required field

Page is here: http://warhammerworld.games-workshop.com/events-hall/reserve-a-gaming-table/

Customers using Firefox or Explorer don’t get the drop down calendar option to select a date, which is given when using Chrome. Manually inputting generates an error message, then won’t send the form.

We have tried dd/mm/yy, mm/dd/yy, dd/mm/yyyy and mm/dd/yyyy but none are accepted.

Is there a fix to get the calendar drop down working on these browsers, or a date format we can specify our customers to use?

https://wordpress.org/plugins/contact-form-7/