Calculating standard deviation/average of differences between a list of DateTimes, and determining if a given diff is unlikely.

date_diffs.php

<?php
/**
 * Yields the list of differences between a list of datetimes.
 * @param array $dates An ordered list of datetimes of which to find the
 * differences.
 * @return array The differences between each sequential datetime in $dates.
 */
function date_diffs($dates) {
    $diffs = array();

    for ($i = 1; $i < count($dates); $i++) {
        $diffs[] = $dates[$i]->getTimestamp() - $dates[$i - 1]->getTimestamp();
    }

    return $diffs;
}

$diffs = date_diffs($dates);

$avg = array_sum($diffs) / (count($dates) + 1);
$stddev = stats_standard_deviation($diffs);

if ($some_time_diff > $avg + 3 * $stddev) {
    // There must be something wrong!
}