dd/mm/yyyy
There should be a dropdown for it anyhow. Which web browser are you using pretty sure there was problem for this before on firefox?

This is what @PKR said before for the same problem:
‘You seem to be using an old browser .. else it would just give you a date format and a drop down calendar.

It was a hard decision, but maintaining support for old browsers get exponentially time consuming, upgrading to a latest browser will not only improve your browsing experience, but will keep your security up to date as well.

Having said that, the format we expect is “yyyy-mm-dd” since it is the most unambiguous. Next in line would be “mm/dd/yyyy” (notice the forward-slash vs hyphens)

– See more at: https://hasitleaked.com/discussion/topic/date-format/#sthash.LRdenSXW.dpuf‘

Hope it helps