The clock is useful, if not downright essential, on the PCM-M10 because it names the files for the day you record them, and of course they are time-stamped like other Windows files. If you leave it unset all your files are going to have duplicate--[EDIT: actually, just similar dated, as someone pointed out below]--names, and if, like me, you sometimes are tardy about uploading them, it sure is nice to have the date to jog your memory.
If it's not working I would consider it defective and get it replaced. There's no reason to live with that defect. Mine has never had a problem.
-------------
And meanwhile, on Rich62's battery box question...a battery box is all you'll ever need for loud music. Save the extra $$ for beer or other experiential enhancements.