TIME ZONE SQL

Hi Pls see this Query

I have following fields in my table "customer_events"

a) time_difference (which has values like -05:00 , +05:30, +00:00 etc)
b) start_time (has value like 11:05, 10:00 etc)
c) send_before_time (has value like 00:05, 00:10 etc)

select 
((CURRENT_TIME(0) AT TIME ZONE "interval" (time_difference)) BETWEEN 
(start_time::time - send_before_time::time) 
and start_time::time) as yesno,
current_time(0), 
CURRENT_TIME(0) AT TIME ZONE "interval" (time_difference), 
(start_time::time - send_before_time::time) as difference, 
time_difference
from 
customer_events


MY PROBLEM:

This query runs fine when i have 
time_difference value like +5:30 +5:00 i.e. works fine for positive values
but failes for negative values i.e. -7:00, -6:00 etc

I don't know WHY WHY... pls help
I am helpless.

do reply back....

Regards,
Raman Garg