I use the following code in conjunction with the legacydatetimefields module to allow the administrator to select a date:
$fields->addFieldToTab("Root.Content.Times", new PopupDateTimeField('RankingDate', 'Ranking Date'));
When a page is create that field is blank. I want the current date to be placed in that field when new pages are created but after the page is created I want the administrator to be able to change it to any date.