PHP OADate formatting - To and From...

oadate.php

<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');

if ( ! function_exists('toOADate'))
{

    function toOADate($timestamp) {
        $from_date = new DateTime("1899-12-30");
        $to_date = new DateTime($timestamp);
        $interval = $from_date->diff($to_date);
        return $interval -> days;
    }
    
    function OADate_to_str($duration, $format="Y-m-d") {
        $adate = new DateTime("1899-12-30");
        $adate -> modify( "+{$duration} day");
        return $adate -> format($format);
    }

}

https://gist.github.com/DaveRandom/ad5872a3fdb4e5126f38

@seanbamforth and @DaveRandom, thanks for the gist. When using it to populate a Google Sheet, minor deviations seemed to occur:

2020-06-25 18:00:00 in PHP becomes 2020-06-25 17:59:32 in Google Sheets. These missing 28 seconds "might" occur because DateInterval does not support leap seconds?

But also 2020-10-29 18:00:00 becomes 2020-10-29 17:19:32, which I cannot explain...

Using https://github.com/PHPOffice/PhpSpreadsheet/blob/0ed5b800be2136bcb8fa9c1bdf59abc957a98845/src/PhpSpreadsheet/Shared/Date.php#L240 prevented these deviations