Other issue with Now date in comparison (formula/filter)

[Hesaf]

We have another issue wrt the now usage in formulas/filters:

Required:
"Filter that includes task with finish date less than or equal to today"

Initial thought for filter:
IIf([Finish]<=Now(),True,False).
But this doesn't work. It will not mark tasks 'True' which end on the same day as returned by Now() although <= is taken as comparison.
-------------------------
Analysis:
PC clock date & time : Wk27.4 - 13.00
Now() date & time returns : Wk27.4 - 13.00
So Now() accurately describes the current date & time.

Task entered with finish Wk27.4 supposedly is stored internally as : W27/4/09 8:00 AM (start for working day)
If the filter would take hours into account, the '<=' should have worked properly....... (but take into account hours is actually not what we want to compare on).

If the filter would just compare the date, the '<=' should have worked.. Why isn't it working ?
----------
After some experimenting :
IIf([Finish]<=Now()+0.5,True,False) seems to work.
It also correctly seems to mark tasks 'False' finishing Wk27.5.

What is the explanation for this issue as it seems not to be logical what is happening?