php - MSSQL Server 2008 Conversion failed when converting date and/or time from character string Error

I am currently receiving the MSSQL Server 2008 error:Conversion failed when converting date and/or time from character string.

I've checked the following threads but am still having no lucks:

Conversion failed when converting date and/or time from character string while inserting datetime Conversion failed when converting date and/or time from character string in SQL SERVER 2008

Here is the query (I'm using PHP's PDO SQLSRV):

SELECT ISNULL(MYDATE,'None') AS MYDATE, 
ISNULL(CAST(rnm AS NVARCHAR(50)),'None') AS Rnm, 
DATENAME(dw, MYDATE) as nameOfDay, 
FROM tab1
INNER JOIN rnm ON (tab1.rte = rnm.rte)
WHERE DATEDIFF(S, '1970-01-01', MYDATE) >= :startDate
AND DATEDIFF(S, '1970-01-01', MYDATE) <= :endDate
GROUP BY MYDATE, CAST(rnm AS NVARCHAR(50)) WITH ROLLUP
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The MYDATE field is of the typedatetime in MSSQL. I have tried casting each of the MYDATE variables and the1970-01-01 all as datetime, but I still receive the error.

The "rnm" is cast as VARCHAR because it's an old DB using the "text" type still, unfortunately.

Thanks.

---update:

$startDate = strtotime($_GET['startDate'] . " 00:00:00");
$endDate = strtotime($_GET['endDate'] . " 23:59:59");
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The above is then bound to the prepared variable viabindParam().