How to simplify if else with four different conditions

How can I simplifyy this code of mine?

You can save the result of Carbon::*() into a variable and call its members.

if (empty($startDate)) {
    $carbon = Carbon::now();
} else {
    $carbon = Carbon::createFromFormat(...);
}
$post->start_at = $carbon;
if (empty($endDate)) {
    $post->end_at = $carbon->endOfDay(); 
} else {
    $post->end_at = Carbon::createFromFormat(...);
}