date timezone question

[P]

Q1. Why is there a difference in the parsing of $TZ
    and --date ... timezone ?
Q2. Why is a warning not printed when an invalid $TZ is set?

Details:

I needed to organise a meeting for 09:00 in San José
and so to figure out the time here (Irish Summer Time) I did:

$ date --date "09:00 PST"
Wed Mar 31 18:00:00 IST 2004

Cool
right, now I tried to see what time it was there now like:

$ TZ=PST date
Wed Mar 31 15:19:20 PST 2004

that's wrong so I tried

$ TZ=crap date
Wed Mar 31 15:19:20 crap 2004

interesting, crap is treated as UTC?
using tzselect I found the right one to use

$ TZ=America/Los_Angeles date
Wed Mar 31 07:21:51 PST 2004

There is some pertinent info in the docs:

Time zone items
===============


   A "time zone item" specifies an international time zone, indicated
by a small set of letters, e.g., `UTC' or `Z' for Coordinated
Universal Time.  Any included periods are ignored.  By following a
non-daylight-saving time zone by the string `DST' in a separate word
(that is, separated by some white space), the corresponding daylight
saving time zone may be specified.


  Time zone items other than `UTC' and `Z' are obsolescent and are not
recommended, because they are ambiguous; for example, `EST' has a
different meaning in Australia than in the United States.  Instead,
it's better to use unambiguous numeric time zone corrections like
`-0500', as described in the previous section.

cheers,
Pádraig.