Search between dates won't work properly

I'd like to search for invoices in the database, having the date range
set (from, to). The problem is that invoices whose issue dates match
'from' or 'to' dates will not show up in results.

For example:
Created an invoice today ("2009-02-09"), and I'm searching by
between from and to, these are the params:

"filter"=>{"from"=>"2009-02-09", "to"=>"2009-02-09"}

Is the issue date a date column or a datetime column ?
if you have a datetime column containing 2009-02-09 14:00 and compare
it to 2009-02-09 then the 2009-02-09 is turned into 2009-02-09 00:00
which can mess things up for you.