Prepare GPX for Strava (see https://support.strava.com/hc/en-us/articles/206811950-How-to-import-a-GPX-File-as-a-Strava-Route for details).

gpx4strava.php

#!/usr/bin/env php
<?php

$file = $argv[1];

$date = (new DateTime())->modify('-1 day');

$doc = new SimpleXMLElement(file_get_contents($file));

foreach ($doc->trk->trkseg->children() as $k => $trkpt) {
    $trkpt->time = $date->format('Y-m-d\TH:i:s\Z');

    $date = $date->modify('+1 second');
}

// Output can be redirected by the user.
echo $doc->asXML();