First of all, this is the iControl forum, so in the future you might want to post iRules questions to the iRules forums. You'll likely get quicker responses as some iRules users don't monitor the iControl forums.
With that being said, can you provide some code? The usage for the "HTTP::cookie expires" command is:
HTTP::cookie expires [seconds] [absolute | relative]
I don't see in your code where you are specifying the cookie name you are referencing. There can be multiple cookies, and to get or set the expiration on the cookie, you'll need to specify the name. If you could provide some code, I might be able to comment further but without it I'm only guessing.
-Joe
Nimbostratus