if the date is in ISO standard format i.e. "yyyy-mm-dd" then you can verify the date simple condition. as example:
if('2012-10-11' < '2012-10-10')
{
echo "not true";
}
if('2012-10-11' > '2012-10-10')
{
echo "true";
}
if('2012-10-11' <= '2012-10-11')
{
echo "true";
}
if it is not in ISO standard then convert it to ISO standard. By the way MySQL store date as ISO standard so don't worry about it. but if your input box does not receive it in ISO Standard then convert it.