I have the 444100 model and for somw reason i see

Just press some buttons on your Audio Player and Recorder.

If your stated 'make model (what it is)', it would help a bit!!

Or you could Google

make mode manual

or

make model change from military time.

You know that Google knows everything right?

..

you have it set on 24 hour time instead of 12 hour time
it will change over at midnight to military time as that is when the 24 hour period ( military time ) starts

Casios have 2 time modes.The 12 hour mode (am,pm) and the so called "military time" 24 hour mode.When your set time,date,year there is an option 12H or 24H.Choose the 12H by pressing the lower right button when in setting procedure,I say again.Press tthe upper left "ADJUST" button to finish the procedure.

12 hour is regular time and 24 hour is military time, example for military time would be 06:00P.M. regular time or 1800 for military time.

This is for the Bose Wave Radio. If you have a different model just let me know. I also included a link to the manual just in case you don't have one

Hope this helps.

To set AM/PM (12 hour) or Military (24 hour) time - With the system off, hold down Alarm Setup and press On/Off to switch between AM/PM (12 hour) and Military (24 hour) time formats.

http://www.bose.co.uk/GB/en/images/Wave%20Radio%20CD_tcm6-4104.pdf

military time:
1. When setting the time on your watch, IF you watch can change to Military time(and most can)
2. when you are adjusting the hours go through a full 24 hour cycle. You'll notice that the 2nd time through it is on military time.. 3rd time through back to 12 hour day AM/PM cycle .. 4th time back to Military.
So you can do this as many times as it takes to get to the right time ..
In 24 hour time you will see an "H" displayed where you normally see and "A" for AM and "P" for pm.

This may or may not work. The 40009 and 40010 are almost identical. The 40009 doesn't mention military time and the 40010 does, so you might want to what the manual says. 40010 manual and 40009 manual. According the 40010 manual, is says:

Displaying the12/24 hour, Date, Month and Year:
Display the 12/24 hour by hold the "+" button for 2 seconds. To display the date, month and year by press the "-" button, the year appears and then if pressed again, the month and date (eg. “12 31” means Dec 31) appears.

So maybe that will do. I have a 40010, but my manual is different from the one on the website. Its DST button and not Alarm Set button to change the time. So maybe try holding down different buttons for a couple of second see if one lets you change it between 12 and 24 hour. Good luck, hope this helps.

hit mode three times until TUES flashes and the seconds. Hit reset until the hours flash and hit start to change them. The watch goes from AM hours to PM and then back through AM and when it cycles through the PMs for the second time, they will alternate 12/24 hour time.