Clock setting in JVC car stereos is very similar over quite a range of models so you could maybe try these two procedures which are used in the S13 & KDG230 respectively.

(1) You press and hold SEL (select) for two seconds until the MENU comes up.

Let go of SEL.

Press the left and right arrows on the side of the radio, where you normally change the station or skip tracks.

There's all sorts of different menus, including Hour and Minute to change time (you have to confirm hour/minute, then go back to hour/minute to do both) as well as the annoying DEMO function so just turn the knob to the left to turn it off while you are in the menu.

For each function you change to what you want, you press SEL again to set it.
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(2) First, you need to press and hold down the SEL button near the volume knob, you may find it best when in the CD mode to do this.

Once DEMO appears, use the next track button (on the circular pad next to the volume knob) to track your way to CLCK H -

Then you use the volume knob to change the Hour.

Follow the steps again to get to CLCK M for the minutes.

Finally just press (don't hold) Select again to finalise.