PL/SQL :: Dates And Times In A Subquery With A Group By

I have some data that I need to group to the Month, Day, Year, Hour and minute in a subquery. Then I need to summarize it to the Month and year in the parent query.If I try to group by the field itself, it is not taking it down to hour and minutes - just the day, so I am losing records.if I do a TO_char (visitdate, 'DD-MON-YY HH:MI AM') in the subquery, then the main query no longer sees it as a date, so cannot do my TO_CHAR(VISITDATE,'MON-YYYY') in the parent. I could parse out the pieces using string manipulation, but that seems rather silly.Is there a way to keep as a date in my sub query and then convert to a string?

it looks a little like this, with some other fields that I have to max, sum ...

visit     provider     person     visitdate
1     2     1     12/20/2012 10:30
2     2     2     12/20/2012 10:30
3     2     5     12/20/2012 11:30
4     3     3     12/21/2012 11:30
5     3     4     12/21/2012 11:30

I need to boil this down to

provider     visitdate
2     12/20/2012 10:30
3     12/21/2012 11:30
2     12/20/2012 11:30

Then I use that in a subquery where I use just the month and year TO_CHAR(VISITDATE,'MON-YYYY') AS APPT_MO_YR right now if I do a group by visitdate on the subquery it returns

provider     visitdate
2     12/20/2012
3     12/21/2012

even if I do a group by to_date(visitdate, 'DD-MON-YY HH:MI AM') it is still returning :

provider     visitdate
2     12/20/2012
3     12/21/2012