Agreed, looks like a bug, but could not reproduce in this example:
create table fdhista (rowtype tinyint, zdate date);
insert into fdhista values (1,'2013-09-13'),(1,'1970-12-31'),(3,'2013-09-14'),(3,'1970-12-30'),(3,'1984-06-24');
When I then run
select rowtype, min(zdate), max(zdate) , count(*) from fdhista group by rowtype;
I get
+---------+------------+------------+------+
| rowtype | L1 | L2 | L3 |
+=========+============+============+======+
| 1 | 1970-12-31 | 2013-09-13 | 2 |
| 3 | 1970-12-30 | 2013-09-14 | 3 |
+---------+------------+------------+------+
And with the restriction
select min(zdate), max(zdate), count(*) from fdhista where rowtype=3;
I get
+------------+------------+------+
| L1 | L2 | L3 |
+============+============+======+
| 1970-12-30 | 2013-09-14 | 3 |
+------------+------------+------+
Everything looks fine, but I am running the latest MonetDB version here.
So first, update to the latest MonetDB version Jan2014. Second, please report MonetDB bugs in the bugtracker at http://bugs.monetdb.org/. Please make sure to include enough information into the bug report so that it can be reproduced, I had to guess the schema and and come up with sample data in this case.