JSFiddle - React, Tailwind, and code Playground

JavaScript

function convert_time(duration) {
    var total = 0;
    var hours = duration.match(/(\d+)H/);
    var minutes = duration.match(/(\d+)M/);
    var seconds = duration.match(/(\d+)S/);
    if (hours) total += parseInt(hours[1]) * 3600;
    if (minutes) total += parseInt(minutes[1]) * 60;
    if (seconds) total += parseInt(seconds[1]);
    return total;
}

alert( convert_time( 'PT2H5M15S' ) );