function timeConversion(str: string): string {
    const pad = (n: number) => n.toString().padStart(2, '0')

    // 0. read 12h
    let h = Number(str.substring(0, 2))
    let m = Number(str.substring(3, 5))
    let s = Number(str.substring(6, 8))
    let p = str.substring(8, 10)

    // 1. convert to 24h
    if (h == 12) h = 0
    if (p == 'PM') h += 12

    return `${pad(h)}:${pad(m)}:${pad(s)}`
}