TAXII X headers

All,
In the spec we defined two headers `X-TAXII-Date-Added-First` and `X-TAXII-Date-Added-Last`.  My question is, should these be the time the object was added to the server or the time it was added to the collection that you are pulling from...
When I wrote this originally, I was thinking about the time it was added to the server.  But now that I have working code and am trying to implement this feature, there is a difference between when an object is added to a server and when it was put  in a collection. Some times these will be the same time, other times, an object may be added to a collection after some sort of analysis, which means it was already in the server.
Bret