为什么这个SQL语句不会使用字符串（mm / dd / yy）并将其转换为天？

In my database, I have varchar values that signify dates - however I cannot input them into the database as dates. So, I am trying to pull them from the database and use them to calculate days.

Format in table:

mm/dd/yy

My SQL Statement (in PHP):

$conn = oci_connect("user", "pass", "(description=(address=(protocol=tcp)(host=host)(port=1533))(connect_data=(service_name=sid)))");

$sel = "select RECEIVED from INTOXDM.JOINT_USE where RECEIVED is not null";
$par = oci_parse($conn, $sel);
$exe = oci_execute($par);
$fetch = oci_fetch_all($par,$array);
echo $fetch;

$arraynum = 0;

while ($array) {

    $arraynum = $arraynum + 1;
    $date = $array[RECEIVED][$arraynum];
    $today=date("m/d/y");
    $now=strtotime($today); // or your date as well
    $your_date=strtotime("$date");
    $DAYS=($now - $your_date)/(60*60*24);


    $upd = "update INTOXDM.JOINT_USE set DAYS = '$DAYS' where RECEIVED = '$date'";
    $pars = oci_parse($conn, $upd);
    $exe = oci_execute($pars); }

This code seems to work for some of the varchars, however many others are passed as a ridiculous value (16k days). This is because the date is being saved as 12/30/1969, I believe. This signifies that I have a problem with my date coding, but I cannot seem to figure it out.

Perhaps you should do the calculation in Oracle?

select trunc(sysdate - to_date(RECEIVED, 'MM/DD/YY'))
from INTOXDM.JOINT_USE
where RECEIVED is not null;

You can even do the update this way as well:

update INTOXDM.JOINT_USE
    set DAYS = trunc(sysdate - to_date(RECEIVED, 'MM/DD/YY'))
    where RECEIVED is not null;

The moral of the story is that dates should be stored in databases using the native date/time formats.