I am new to typo3 and I created a simple contact form using powermail and I took a date picker of powermail. In that, the default placeholder is dd/mm/yyyy. But I want a different placeholder. So how I can do that?
A placeholder can not be shown in a HTML field of type date. The browser (depending on your OS and country settings) will show a placeholder with a format to help you to fill out the date field. If you really need a different placeholder, you could take some JavaScript to switch the type of the field from text to date on focus (or similar stuff). But this seems to be very "hacky" in my eyes.