time_zone and ERROR 1146 Table '.' doesn't exist

Hi,

I need the unix_timestamp of the Nth day of the year, at 00:00:00.

I have tried this function :
CREATE FUNCTION jour (j integer ) RETURNS int(11)
-> DETERMINISTIC
-> BEGIN
-> RETURN unix_timestamp(concat(year(now()), "-01-01 00:00:00")) + 24 *
3600 * (j-1) ;
-> END

It works ... but I need the result in UTC, and the server is in WET, so
the result is not the expected one :

select jour(65) ;
+------------+
| jour(65) |
+------------+
| 1173135600 |
+------------+

If I use :

set @@time_zone="+0:00"; select jour(65) ;
+------------+
| jour(65) |
+------------+
| 1173139200 |
+------------+

this is the expected result. But users (me !) may forget to set the
time_zone, so I try to put the instruction in the function, like this:

CREATE FUNCTION jour (j integer ) RETURNS int(11)
-> DETERMINISTIC
-> BEGIN
-> SET @@time_zone="+0:00";
-> RETURN unix_timestamp(concat(year(now()), "-01-01 00:00:00")) + 24 *
3600 * (j-1) ;
-> END

And now I get :

select jour(65) ;
ERROR 1146 (42S02): Table '.' doesn't exist

Gasp ! Any idea of the problem, and how to fix of it ?

TIA,

--
Philippe Naudin