Odd issue with WHERE clause

Greetings All,

I'm having an odd issue with a WHERE clause that I've been working on in a Record Set, $upperdate displays as 2011-02-28, $lowerdate as 2011-02-01, but,

WHERE visitdate >= $lowerdate AND visitdate <= $upperdate

returns no values, and

WHERE visitdate >= $lowerdate AND visitdate <= '2011-02-28'

returns the appropriate values,

Here is the code for $upperdate and $lowerdate:

******************************************
$datemonth = date ('m');
$dateday = '01';
$dateday1 = date ('t');
$dateyear = date ('Y');

$lowerdate = date("Y-m-d",mktime(0,0,0,$datemonth, $dateday, $dateyear));
$upperdate = date("Y-m-d",mktime(0,0,0,$datemonth, $dateday1, $dateyear));
******************************************

I've also tried the following things to check functionality with no success:

$upperdate = '2011-02-28';

$dateday1 = '28';

BETWEEN...AND... behaves the same as... >=  AND <=

Thanks for the help in advance!