@Peter Coles wrote "HTTP Cookies: What's the difference between Max-age and Expires?" in 2009, but he has updated the article later and there is the answer for wether to use "expires" and/or "max-age" when you set cookies.
What is "max-age"?
Max-age sets the time in seconds for when a cookie will be deleted (use this, it’s no longer 2009) [...] expires was deprecated and replaced with the easier-to-use max-age
About browser compatibility, he states:
Unfortunately, none of the current versions of Internet Explorer support max-age
Not sure when Peter updated his post last, but he closes it with
UPDATE: just use Max-Age, the web has improved since this was written.
If you don't write IE7 apps use "max-age"!