To move the clock in the Hyundai Sonata NF salon out of a 12-hour clock to a 24-hour mode, you must both press and hold both keys at the same time until the display starts blinking "24:00". After that, you will only need to set the time.
If this is not the case (usually in the Sonata NF), try and hold the R (Reset) button for a few seconds.
