I am thinking reserved word for cookie name

I wonder if this is a bug in the version of LTM you're trying with HTTP::cookie expires. Which LTM version are you testing on?

I just tested on 11.0 and 10.2.2 and both work as expected:

when HTTP_RESPONSE {
   log local0. "Inserting PD-H-SESSION-ID"
   HTTP::cookie insert name "PD-H-SESSION-ID" value test path "/"
   if {[HTTP::cookie exists "PD-H-SESSION-ID"]} {
      log local0. "Set-Cookie (pre): [HTTP::header values Set-Cookie]"
      HTTP::cookie expires {PD-H-SESSION-ID} 30 relative
      log local0. "Set-Cookie (post): [HTTP::header values Set-Cookie]"
   }
}

And here's the log output:

< HTTP_RESPONSE>: Inserting PD-H-SESSION-ID
< HTTP_RESPONSE>: Set-Cookie (pre):  {PD-H-SESSION-ID=test;path=/;}
< HTTP_RESPONSE>: Set-Cookie (post): {PD-H-SESSION-ID=test;expires=Tue, 15-Nov-2011 03:10:02 GMT;path=/;}

Aaron