Can anyone tell me why this does not work?

tar cfz "bigfile_`date '+%R-%F'`".tgz bigfile.matt
bigfile_16: Unknown host
tar: bigfile_16\:55-2009-02-18.tgz: Cannot open: Input/output error
tar: Error is not recoverable: exiting now

I'm running a script in cron that creates very large log files every four hours, so at the end of the script i want it to tar the log file and name it with a date / timestamp as there will be lots and lots of these files if i ever get it to work.

I suspect it has something to do with the date format having strange characters and or spaces ??

Thanks