This doesn't seem to be the kind of thing you can change using
hook_form_alter() unless I'm missing something.
The label of the "From" field can be any value you want, but the "to" field, if you have one, defaults to "to", see
modules/date/date_elements.inc, line 369:
$element[$to_field]['#title'] = t('to:');
so what's the correct way to change that to a value of my choosing, i.e. "Expires"? Via the Theme?