In the select statement I changed the table name where your retrieving the date.
Thanks Again
UPDATE AE SET AE.count_fact = dtCounts.count_fact
FROM #tmp_ar_exception AE INNER JOIN
(
SELECT DISTINCT #tmp_ar_exception.dt_id, COUNT(F_AR_Detail.date_id) AS count_fact FROM #tmp_ar_exception LEFT OUTER JOIN
F_AR_Detail ON #tmp_ar_exception.dt_id = F_AR_Detail.date_id
GROUP BY #tmp_ar_exception.dt_id
) dtCounts on AE.dt_id = dtCounts.dt_id