Daylight Savings Time

Hello List:

I need to know if there is a convienient way of establishing whether DST is
active within a function dealing with adjusting timestamps to other time
zones. The problem is that if I have the following timestamp:

'04/21/2006 17:05 EDT'

and I use the timezone() function in the following manner:

return (timezone ('CST', '04/21/2006 17:05 EDT')

I get a two hour difference in time. Note that neither of the two arguments
are hard coded as this example. The "CST" value is stored in the customer
profile because that is their time zone and the timestamp is generated from
argeuments passed into the function.

This is:
rnd=# select version();
version
--------------------------------------------------------------------------------------------------------------
PostgreSQL 7.4.6 on i686-redhat-linux-gnu, compiled by GCC gcc (GCC) 3.2.3
20030502 (Red Hat Linux 3.2.3-49)

TIA