In appendix C of F&O, http://www.w3.org/TR/xpath-functions/#error-summary FORG0008 is defined as: err:FORG0008, Both arguments to fn:dateTime have a specified timezone. However, this error is only raised if the two arguments have _different_ timezones. "If the two arguments have different timezones, an error is raised:[err:FORG0008]"
The summary of errors in appendix C is non-normative, and many of the descriptions are extremely terse, for example "Invalid lexical value". I'm not sure that improving these descriptions by erratum is a useful service to our readers.
I found it confusing, and just thought it was worth pointing out.
Following direction at the joint WG meeting of 2008-03-04, I have drafted an editorial erratum (E18) which changes the text to The two arguments to fn:dateTime have inconsistent timezones I am marking this as FIXED. Tim Mills, as the originator, if you agree with this resolution please change the status to CLOSED.
Thanks Mike.