Cookie Expire Time - Client or Server?

When setting the expire time of a cookie in PHP like: setcookie ("TestCookie", $value, time()+300); /* expire in 5 minutes */ the time is obviously relative to the current time on the server. If the client's clock isn't in sync with the server, isn't there a problem as to when this cookie will actually expire? What I want is for the cookie to expire after 5 minutes from the time it was set, but if the client's clock has a different time than the server's my guess is that the cookie will expire either faster or slower than the 5 minutes that I want. Can someone confirm my suspicions here. If I'm correct, does anyone have a workaround? Thanks!