Using httpmonitor.jar from the NetBeans dev build 
200302270100, it appears that Monitor records header 
values by always calling request.getHeaderNames() and then 
request.getHeaders(name).

This is correct and doesn't create a problem in most 
instances.  However, if getHeaders(name) is called with 
Sun ONE App Server, the server will parse any header 
values as if they were comma-separated values.  So, even 
if there's only one header for a given header name, it may 
return multiple values after parsing that.  There's 
nothing in the servlet spec that suggests what S1AS does 
here is incorrect.

However, in the case of date headers because Monitor 
always calls getHeaders(name), S1AS will parse the value 
of the date header as if it were a comma-separated list of 
values.  Since RFC 1123 date format is like "Mon, 06 May 
1996 04:57:00 GMT", this will always lead to date headers 
being incorrectly recorded by Monitor as multiple values.

Since clients are unlikely to set multiple values for date 
headers, it would be helpful if Monitor would treat known 
date headers specially and use getDateHeader(name) instead 
of getHeaders().

Known date headers that I can think of are:

  * Date
  * If-Modified-Since
  * If-Unmodified-Since
  * Retry-After
  * Last-Modified