I urgently need an owners manual for a Pioneer DEH-2100R Car cd Radio. Is there any available in download format ?

Try here, although you may be asked to supply personal details. Be wary of unwanted adware if you don't make the right selections. AVG online scan says this site is safe.

http://www.u-manual.com/download-manual-now-for-free-10/166712/car-radio-pioneer-deh-425.html#.UwXP2GKSxDQ

Here is a link to the owner's manual for the stereo which will tell you how to set the time:
http://a248.e.akamai.net/pix.crutchfield.com/Manuals/130/130DEH1800.PDF

This is a link to Pioneer for your DEH-2000MP Owners Manual. Look on page 12 for accessing Functions to set your clock.
http://www.pioneerelectronics.com/StaticFiles/Manuals/Car/DEH2000MPOwnersManual1019.pdf

Here is your owners manual for FREE!!!!
http://www.pioneerelectronics.com/pio/pe/images/portal/cit_11221/192094167DEHP7700MPOperationManual.pdf

I can scan and e-mail any pages you may need.

http://audio.manualsonline.com/manuals/mfg/pioneer/fmam_deh1800.html

I need a manual for my DEH-P2900MP CD Receiver.

The unit is in a car that we have just purchased and the manual for the radio was not with the vehicle.

http://www.generalmanual.com/Players/Pioneer-DEH-1600-In-Dash-CD-Receiver.htm

1.press source button and hold until unit powers off
2.press audio and hold until clock appears
3.press -> <- arrows to select hours from minutes
4.0up and down buttons to change the time.

manual can be found at above link ,hope this helps 
Harry

This link will take you to the Pioneer page with the link for both the Owner's Manual and the Installation manual of your specific model.

http://www.pioneerelectronics.com/PUSA/Products/CarAudioVideo/Source/CD-Players/DEH-P490IB?tab=D

Clicking on the link will ask you to register (no cost involved) and then you can download or view the manual.

Hope this helps