The documentation says:
* placeholder - The default comes from the htmlform-(date|time|datetime)-placeholder message.
and the code does set the placeholder:
if ( $this->mPlaceholder === '' ) { // Messages: htmlform-date-placeholder htmlform-time-placeholder htmlform-datetime-placeholder $this->mPlaceholder = $this->msg( "htmlform-{$this->mType}-placeholder" )->text(); }
However, these are not shown, as can be seen e.g. in the form on Special:AbuseLog.