Why doesn't this code render the time in addition to the date?

Why doesn't this code render the time in addition to the date?

This code renders the date (month, day, year) but not the time. It is
being used on a php upload site. I am grateful for any suggestions. Thank
you.
<time datetime="<?= date( 'm-d-Y', strtotime( $file->added ) ) ?>"><?=
date( 'm.d.Y', strtotime( $file->added ) ) ?>