How to get date format DD/MM/YY for UK.?

I am trying to fill in a form but it tell sme that the date format is incorrect for 17 of the 27 entries.

I type 12/07/17. Sometimes it enters and other times it does the calculation.

Do you check this settings?

Yes I did. Set them to UK. The date format offered is D/M/Y;D/M;D-M.

The D/M/Y acceptance pattern should already allow you to enter dates as DD/MM/YY.
Is there a custom macro connected to your input field which checks the validity of the input?