In that case, I would use the following expressions as calculated fields in the query:
startOfDay: DateValue(Now())
endOfDay: DateAdd("s",-1,DateAdd("d",1,[startOfDay]))
Where do you want to use it?
In a query, you could use each line as an expression to create a calculated field for each value.
On a form, you could use:
= GetCurrentDayDates()(0)
and
= GetCurrentDayDates()(1)
as the Control Source properties of two unbound textboxes.
Strange behavior is often a symptom of corruption.
Perhaps delete the list box and recreate it or even replace the entire form.
I'm sure a search of the forum would give much advice on corruption.