SQL & PL/SQL

Hi,
I need to write a function which has two input parameters.
Parameter1 => Date data type, value is in GMT.
Parameter1 => String value which indicates the time offset and it has the following format.

GMT(sign)HH24Mi
for examples:
GMT+400 , GMT-1400

I need add the offset (param2) to the date (param1) and returns as the output.

I wrote a simple code but can't get it to work. I would like to know if there is an another way to do it.

create or replace
Function         F_FUNCTION_NAME(dt date,timeZoneOffset   in   varchar2)
return date
As
    offset  varchar2(9);    
Begin
    Offset := (To_Char(Substr(Substr(Timezoneoffset,4),0,Length(Substr(Timezoneoffset,4))-2)||':'||Substr(Substr(Timezoneoffset,4),-2)));
    return Dt + interval Offset hour to minute;
End;

Thank You.