If this is a BOSE HU and is equipped with RDS (radio data system) you can find a station that broadcasts radio data including current time. With the radio on press and hold the hour or min button. The display will show the current stations time, continue to hold and it will change the time to the stations time data information. I had to figure this one out myself and that is how I did this.
This too had me befuddled and having the wrong time on the clock had me obsessed.
When I asked for local help when the manual instructions didn't work for me 'non Cadillac people' got that sad looked that that said
..."you are going to pay..its a CAD and electrical"
My CAD is a 2003 STS and has been great. Still barely broken in at 70KMs. It had to be a user problem! Today I had an epiphany.
It seems that I can't change the clock time if the radio is on. If it is on, the clock only recognizes an RDS reset for the clock.
THAT is not in the manual!
After getting so much from this forum, I hope that I gave something back today to someone.
Just turn off the radio and your H/M button will work for you.
Had the car almost a year and half and gave up on it. I'm goona try it with it off and see if that does anything. Couple stations tell you artist-song title but no time, so could never switch it. Just learned to live with it. Be nice if it works.