wp-convert-time-to-iso8601-duration.php

<?php
/**
 * Convert time for scheme.
 *
 * @param int $seconds Time in seconds.
 *
 * @return string
 */
function convert_time_to_scheme( $seconds ) {
  $days    = floor( $seconds / DAY_IN_SECONDS );
  $seconds = $seconds % DAY_IN_SECONDS;

  $hours   = floor( $seconds / HOUR_IN_SECONDS );
  $seconds = $seconds % HOUR_IN_SECONDS;

  $minutes = floor( $seconds / MINUTE_IN_SECONDS );
  $seconds = $seconds % MINUTE_IN_SECONDS;

  return sprintf( 'P%dDT%dH%dM%dS', $days, $hours, $minutes, $seconds );
}