Check the time to make sure that it is correct. You may have accidentally mistaken AM for PM, like my friend did. But my friend was being kinda silly, she RESET her Tamagotchi to CHANGE the TIME!
Sounds like you have it set to the wrong time. You can change the time from 24 hour military time to rregular AM and PM by pushing button A when you have the time screen up.
When you want to set the time, press button A to make it AM/PM, and keep holding button A while you push button C to make it go to the time set option. Each time you push button A with the time screen up, it changes from 24 hour to AM/PM and back.