Updating timestamps

Is it possible to update a timestamp with Postgresql?

I've looked around with google and it looks as though this should work:

exec sql begin declare section;
char api[18];
char expiration_time[80];
exec sql end declare section;

memcpy(api, ap->api, sizeof(api));
sprintf(expiration_time, "%s + \"interval\"('0.01sec')",
ap->expiration_time);

puts(expiration_time);

exec sql update accesspoint
set sequence_number = expiration_time = :expiration_time,
where api = :api;

However, I just get this:
2005-08-02 10:37:41.198479 + "interval"('0.01sec')
'ERROR: invalid input syntax for type timestamp: "2005-08-02 10:37:4

-Nigel