I thought I remembered this actually being a behavior I'd seen, but I must have misremembered. We frequently prepare memos and letters for distribution to employees for completion who will only have access to Reader, or other PDF-compatible options on mobile devices. For those who are able to use the signature features on these forms, the default signature format includes the date it was signed, but when we need them to provide dates elsewhere in the form, giving them a date picker instead of a plain textbox requires added steps for every date. Is there a setting I'm missing somewhere?
Standards and accessibility
Your question doesn't really match your title... The answer to the title is you can't. You need to do it manually once the fields have been created. Another option is to use a script, to apply a Date format to all fields that have the word "date" in their name, for example.