From what I remember safari is very strict about the date format (other browsers tend to be very lenient). Safari expects the date to be in the format of Wdy, DD-Mon-YYY HH::MM::SS GMT (Sat, 23-Dec-2006 02:06:57 GMT). I would first double check that your date's in the right format. But unless your using an old or broken version of CGI::Cookie, I expect it's fine.

Other than that, I would check the clock on the client and ensure it's not skewed in some odd way.

Other than that, I would do as the first comment suggested and not really rely on the expiration date -- keep track at the server - that's what sessions are for.