form validation, set_value and initial value

Hello,
It seems impossible to set initial value to a form field when you need to use set_value() function to repopulate the form field with the submitted data, Am I wrong ?
"Placeholder" might be a solution to suggest something to the end-user but when it comes to "date", setting initial value to date-of-the day is so comfortable ...
Help and / or explanations will be apreciated.
Thanks,
Eric