Обсуждение: sum gives different answer

I have two SQL statements that I expect to give (0 rows) as output.
However the first one with 'sum' does not do that.
Why the first SQL statement gives (1 row) and empty?
While the second SQL statement can give (0 row).
I am using postgresql 6.4.2.

First
------
select pin, sum(duration) from tablename where date(start_time)=
(select date 'today') group by pin;

pin|sum
---+---
   |
(1 row)
 

Second
------
select pin, duration from tablename where date(start_time)=
(select date 'today') group by pin;

pin|duration
---+--------
(0 rows)

Could anyone tell me why, please?

Regards
Chai