Can't get time zone information when using mod_plsql?

I have written a function to convert a date into a Unix time stamp. The function is written to work no matter what the current DST status is (e.g. EST or EDT). This is the function:

function unix_time_from_date(in_date in date) return number
as
  ut number     := 0;
  tz varchar2(8) := '';
begin  
  -- Get the local timezone from the passed in date
  -- Assuming the date supplied is for the local time zone
  select
    extract(
      timezone_abbr from cast(in_date as timestamp with local time zone)
    )
  into tz
  from dual;

  -- Get the Unix timestamp
  select
    (new_time(in_date, tz, 'GMT') - to_date('01-JAN-1970', 'DD-MM-YYYY')) * (
    86400)
  into ut
  from dual;

  return ut;
end unix_time_from_date;

This function works great when I execute it from a client like JDeveloper. From what I gather, this is because the client is supplying time zone information to the first query. However, if I use the function from within a procedure that is called from a mod_plsql page, then I get the error ORA-01857: not a valid time zone. This error is being thrown from the new_time function because tz is set to 'UNK'.

So, I implemented a work-around for this problem like so:

function unix_time_from_date(in_date in date) return number
as
  ut number     := 0;
  tz varchar2(8) := '';
begin  
  -- Get the local timezone from the passed in date
  -- Assuming the date supplied is for the local time zone
  select
    extract(
      timezone_abbr from cast(in_date as timestamp with local time zone)
    )
  into tz
  from dual;

  if tz = 'UNK' then
    select
      extract(
        timezone_abbr from cast(sysdate as timestamp with local time zone)
      )
    into tz
    from dual;
  end if;

  -- Get the Unix timestamp
  select
    (new_time(in_date, tz, 'GMT') - to_date('01-JAN-1970', 'DD-MM-YYYY')) * (
    86400)
  into ut
  from dual;

  return ut;
end unix_time_from_date;

Except, this still fails with tz being set to 'UNK'. Does anyone know what could be happening here? Why can't I get the local time zone abbreviation when the function is called from a Oracle Application Server process?

The function as written does not work when the session calling it does not have the time zone information set. Therefore, you need to explicitly specify the source time zone. The following function solves this problem (and corrects the return type):

function unix_time_from_date
    (
      in_date   in date,
      in_src_tz in varchar2 default 'America/New_York'
    )
  return integer
as
  ut      integer       := 0;
  tz      varchar2(8)   := '';
  tz_date timestamp with time zone;
  tz_stmt varchar2(255);
begin
  -- Get the local time zone abbreviation from the passed in date
  tz_stmt := 'select systimestamp at time zone ''' || in_src_tz || ''' from dual';
  execute immediate tz_stmt into tz_date;
  select
    extract(timezone_abbr from tz_date)
  into tz
  from dual;

  -- Get the Unix timestamp
  select
    (new_time(in_date, tz, 'GMT') - to_date('01-JAN-1970', 'DD-MM-YYYY')) * (86400)
  into ut
  from dual;

  return ut;
end unix_time_from_date;

Note the addition of a second parameter to the function. This parameter, in_src_tz, is used to indicate what time zone the in_date parameter is in. The value of in_src_tz should be one of the timezone listed in the tzname column of the v$timezone_names table.

Also, you cannot simply select the value of the tzabbrev column in the v$timezone_names table due to time zone having multiple abbreviations. By using the extract, you will get the current abbreviation with DST factored in.