I have a report that is detailed below, but I need to add a Date Range to get the correct totals.

SELECT T0.workitem_category_tree_value AS 'Category', COUNT(workitem_category_tree_value) AS 'Number of Incidents' FROM HD_workitem_current_view T0 
WHERE ((T0.workitem_category_tree_value LIKE '%CAP%'
OR (T0.workitem_category_tree_value LIKE '%ds%')
OR (T0.workitem_category_tree_value LIKE '%C%')
OR (T0.workitem_category_tree_value LIKE '%C2%')
OR (T0.workitem_category_tree_value LIKE '%Ap%')
OR (T0.workitem_category_tree_value LIKE '%Ac%')
OR (T0.workitem_category_tree_value LIKE '%VPN%')
OR (T0.workitem_category_tree_value LIKE '%P%')
OR (T0.workitem_category_tree_value LIKE '%A%')
OR (T0.workitem_category_tree_value LIKE '%o%')
GROUP BY T0.[workitem_category_tree_value]

I tried to use the article "Modify Reports to Use Date Range" from the site but receive "Conversion failed when converting datetime from character string" errors.
/*SET dateformat ymd     
DECLARE @StartDateTime DATETIME,         
@EndDateTime DATETIME         
SET @StartDateTime = CAST('%StartDateTime%' AS DATETIME)         
SET @EndDateTime = CAST('%EndDateTime%' AS DATETIME)*/

SELECT T0.workitem_category_tree_value AS 'Category', COUNT(workitem_category_tree_value) AS 'Number of Incidents' FROM HD_workitem_current_view T0 
WHERE ((T0.workitem_category_tree_value LIKE '%CAP%'
OR (T0.workitem_category_tree_value LIKE '%Ds%')
OR (T0.workitem_category_tree_value LIKE '%C%')
OR (T0.workitem_category_tree_value LIKE '%C2%')
AND T0.workitem_created_on BETWEEN '%StartDateTime%' AND '%EndDateTime%'))
GROUP BY T0.[workitem_category_tree_value]

I must be doing something stupid - but I cannot figure it out. Any help is appreciated.