timezone help?

I'm a bit new to PG, and having troubles with timestamps.  The docs list:

timestamp [ (p) ] [ without time zone ]    8 bytes    both date and time    4713 BC    5874897 AD    1 microsecond / 14
digits
timestamp [ (p) ] with time zone    8 bytes    both date and time, with time zone    4713 BC    5874897 AD    1
microsecond/ 14 digits 

But an example of how to call to_timestamp either with or without a TZ would help.



My immediate problem is below..

create or replace function get_thermom_type(siteid integer, observationtime timestamp)

select get_thermom_type(1,to_timestamp('01-JAN-2011','DD-MON-YYYY') );
ERROR:  function get_thermom_type(integer, timestamp with time zone) does not exist
LINE 1: select get_thermom_type(1,to_timestamp('01-JAN-2011','DD-MON...
                ^
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.


How do I tell to_timestamp to forget the about time zones?  this didn't work either:

get_thermom_type(1,to_timestamp('01-JAN-2011 HH24:MI','DD-MON-YYYY 13:01') without time zone);
nor a bunch of other attempts.


Thanks for any help,

-Dave