How ISDATE() function work in SQL SERVER

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SQL

SQL-Server-2012

my query is

SET LANGUAGE us_english;
SET DATEFORMAT mdy;

SELECT ISDATE('03/18/2010') AS [ISDATE] UNION ALL
SELECT ISDATE('03-15-2010') UNION ALL
SELECT ISDATE('03.15.2010') UNION ALL
SELECT ISDATE('03/2010/15')

when we execute this query it will return
1
1
1
1

Now my question is
In last select statement is

SELECT ISDATE('03/2010/15')

format of date is MYD. But format of my date is MYD

So according this server should be return 0 instead of 1 but it doesn't return 0.
So Please explain me what is Reason behind it.