Query TIME ZONE

Hi All,
I have a query regarding Time Zone Interval

QUERY:
"select current_time AT TIME ZONE INTERVAL '+5:30'"
it is running fine

BUT
When I take the value of interval from DB table like:

"select current_time AT TIME ZONE INTERVAL time_difference from customer_events"

here : time_difference - is my varchar column in the table customer_events. This gives me errors " parse error at or near "time_difference" at character 43 "

I tried to type cast it time_difference::text etc but it in vain.

Pls help me ... its realy urgent.

Thanks in advance

Raman Garg