First Day of Current Month

Question

I need to return the first day of the current month. I have read I can use:
WITH  MEMBER [Measures].[FirstDayOfMonth] AS 
DateSerial(Year(Now()), Month(Now()), 1)
However, this returns an incorrect format. Can this date member be formatted to yyyy-MM-dd?
I have also read I can use OpeningPeriod but I could not get this to work.
My date Hierarchy is:
[Date].[Year Month Day].[Month]
And my month members are in the yyy-MM-dd format, for example:
[Date].[Year Month Day].[Month].&[2015-02-01]

Answer 1

Thanks Richard,
I am not sure what the "aa" string is for?
Yes, I can get the current date using:
WITH
MEMBER [Measures].[Month Day] AS
FORMAT(Now(), "yyyy-MM-dd")
But I need to return the first date of the current month dynamically. For example, today this value would be: 
[Date].[Year Month Day].[Month].&[2015-02-01]
So I cannot use Parent or FirstChild as the hierarchy will not recognise the
FORMAT(Now(), "yyyy-MM-dd") as a month member as it returns 2015-02-18.
I believe I need to extract the current month day value, subtract this value from the Now() and add 1 (start of month), for example:
StrToMember("[Date].[Year Month Day].[Month ].&[" + FORMAT(Now()-[Month Day]+1,
"yyyy-MM-dd") +"]")
Which will be 2015-02-18 - 18 + 1 = 2015-02-01
Or:
WITH
MEMBER [Measures].[Month Day] AS
FORMAT(Now()-[Month Day Value]+1, "yyyy-MM-dd")
SET [FirstOfMonth] AS
StrToMember("[Date].[Year Month Day].[Month].&[" + [Measures].[Month Day] +"]")
So I need a new member to return the month day value.
Does this make sense? Is there a better way to do this?