with the car on turn the pioneer off and then press and hold the func button until the menu displays
turn the key on but have the CD player off. push and hold the audio button until the clock setting displays
For most Pioneer head units, you can hold down the SRC button until the unit turns off, then hold down the FUNC button until the menu appears. This is usually where you set the time as well as turn on/off M-CD or AUX inputs.
on some radio's you have to take the faceplate off to insert a cd
turn off the unit first hit the clock button then hit the function button then it will flash then its ready for your input "use the arrows when its flashing to change the time".
Turn off the stereo then push the menu button on some models this is the volume dial, if yours pushes in, the it will ask you if you want to set the clock or any other options listed there.
The beeping sound should stop by itself. If the beeping does not stop, then your stereo has a technical issue. Once you get your stereo fixed, it should stop beeping after a little bit.
press src/off until the unit turns off, then press m.c which is the rotary volume control for a few seconds untill settings menu appears
turn off the radio and then press the mute button
Assuming the unit is powered (ignition is "on"), press and momentarily hold the clear square button labeled "source". To turn the unit off, do the same thing. Quickly pressing and releasing the same "source" button will switch the unit between radio function and CD player function.
I suspect you mean why do need to set the clock of the pioneer deh 1550? If this is the case the problem is that there is a wire that comes out of the radio that must have power all the time, that wire is for the memory. You also didn't state if the radio preset stations have to be reset also. This would also be an indication that the memory is not powered up when the key is off.
With stereo OFF...Push and hold the round "D-Pad" for a few seconds, then use the up down left right controls to set time.