Mysterious Mystery -- 2002-11-08/2002-11-09

I have function that returns a table of information about properties. The
data comes from three different tables -- addresses (called PropertyID),
property characteristics, and events concerning those properties (sales,
appraisals, etc.), plus a table that maps one representation of property
types into another. The records are selected on the basis of location
(longitude & latitude), property type, event type, and a range of
event dates (upper and lower date specified). There are tens of millions
of records of all types, and almost any location, property type, event type
and date range will yield records.

The heart of it is a cursor that selects records from joins on this basis:

SELECT <a bunch of fields>
FROM Property d
JOIN PropTypeMap ptm ON ptm.PropertyTypeID = d.PropertyTypeID
JOIN PropertyID a ON a.PropID = d.PropID
JOIN Event e1 ON e1.PropID = d.PropID
LEFT OUTER JOIN Event e2 ON e2.PropID = d.PropID
WHERE
d.LastSaleDate >= @LoDate
AND a.GeoLongitude BETWEEN @LowerLon AND @UpperLon
AND a.GeoLatitude BETWEEN @LowerLat AND @UpperLat
AND ptm.PropCategory = @PropType
AND a.GeoMatch <= @MinGeoQuality

AND e1.EventTypeID = @SaleEventType
AND e1.TransactionType = 'R'
AND e1.EventDt BETWEEN @LoDate AND @HiDate
AND e1.EventAmt > 0

AND e2.EventTypeID = @AssessmentEventType
AND e2.EventDt <= @HiDate
AND e2.EventAmt > 0

Each property has one PropertyID record, one Property record, and N Event
records (average perhaps five).

What is the mystery? If @HiDate, which is the upper end of the time window,
is 2002-11-08 or earlier, nothing is returned. If it's 2002-11-09 or later,
oodles of records are found. I get the same query plan for either one, and
based on the content of the data, they should return almost exactly the same
set of records -- exactly the same set in almost all cases, in fact.

Is 2002-11-08/09 some sort of magic dividing point? I have replicated this
on the large database and on a smaller test version on another SQL Server.
(SQL Server 2000) I dropped the indexes and tried it, and
the same thing happened. This is driving me crazy!