To set the clock, the receiver has to be off. Press and hold the Source button (that's the top of the left-most button) until the receiver turns off. Then press and hold the Multi-Control (the large round dial) until Clock Set is displayed on the LCD screen. Press the Multi-Control. Turn the Multi-Control until the first segment of the time shows the appropriate digit. Press the Multi-Control again to select the next segment and turn the Multi-Control again to set that digit. Repeat the press and turn of the Multi-Control until all segments have the correct digits for the current time. Instead of turning the Multi-Control, you can also pushing it up or down. Press the Back button (to the upper left of the Multi-Control) to return to the previous screen (initial settings).
This is in the manual for the DEH-P4000UB. The manual is on the product page, https://www.pioneerelectronics.com/PUSA/Car/CD-Receivers/DEH-P4000UB#manuals
. Click on the manuals tab if needed. The current link to the manual is https://www.pioneerelectronics.com/StaticFiles/PUSA/Files/DEHP4000UBOperationManual1019.pdf
. See page 27 for these directions. See page 9 for a picture of the buttons on the receiver.
I hope this helps.