Why function return 1

Question

Hello Friend i was doing some exercise on  date time function and face a problem please Help me to solved out this problem

 

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 last select statement is 

 SELECT ISDATE('03/2010/15')  

format of date is   MYD. But i define the format of date MDY 

 

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...........

Thanks for reading my Question..........