Tar -N date error?

[Mike Jeays]

I have trouble with tar's -N format with 5.1 (yes, I am going to upgrade
to 5.3 this weekend).

Commands such as 
tar -cvzf arch.tar.gz -N '2004-09-01' * 
and 
tar -cvzf arch.tar.gz -N'10 days ago' * 
work fine for periods in the recent past (up to about 40 days), but for
more than that, everything in the directory gets archived.  It worked
fine with a version of Cygwin at work on a Windows machine.

Any clues, please?