Hi cimerio,
Is it possible for you to use a hidden field instead, with a substring formula to format the date as dd/mm/yyyy? In order to get the desired format, I would suggest setting the hidden field's default value or formula similar to this:
concat(substring(DateField, 6, 2), "/", substring(DateField, 9, 2), "/", substring(DateField, 1, 4))
Where DateField is the date picker. Hope this helps.