JavaScript

function timeConversion(s) {

let [hours, minutes, seconds] = s.split(':');

if (hours === '12'){
    hours = '00';
}

if (seconds.indexOf('PM')  != -1 && hours < 12 ){
    hours = parseInt(hours, 10) + 12;
}

let conversion = ``${hours}:$`{minutes}:${seconds}`

return conversion.slice(0, -2);

}