JavaScript: convert 12 hours time string to 24 hour time string

JSConvert12to24Time.js

// NOTE: time string must look like this 07:05:45PM or this 07:05:45AM and account for 12:00:00AM and convert 12:00:00pm to 12:00:00
function timeConversion(s) {
    const ampm = s.slice(-2);
    const hours = Number(s.slice(0, 2));
    let time = s.slice(0, -2);
    if (ampm === 'AM') {
        if (hours === 12) { // 12am edge-case
            return  time.replace(s.slice(0, 2), '00');
        }
        return time;
    } else if (ampm === 'PM') {
        if (hours !== 12) {
            return time.replace(s.slice(0, 2), String(hours + 12));
        } 
        return time; // 12pm edge-case
    }
    return 'Error: AM/PM format is not valid';
}

// less lines using array.split/join and modulus (not my solution)
function timeConversionSlicker(s) {
    let AMPM = s.slice(-2);
    let timeArr = s.slice(0, -2).split(":");
    if (AMPM === "AM" && timeArr[0] === "12") {
        // catching edge-case of 12AM
        timeArr[0] = "00";
    } else if (AMPM === "PM") {
        // everything with PM can just be mod'd and added with 12 - the max will be 23
        timeArr[0] = (timeArr[0] % 12) + 12
    }  
    return timeArr.join(":");
}