MS Access issue with "complex" expression

I think you need to use "/" as a separator to match the Format:

CDate(Format([post-period] & "/" & [post-year], "mm/yyyy"))

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BTW, next time please try to paste the actual code into your question instead of pictures.

Having said that, I would suggest the enhancing the expression even further. It looks like you have not specified the day component of the date. I suggest you use dateSerial Access function to properly define the date from chunks:

'1st day of month
DateSerial([post-year], [post_period], 1)

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With the help of dateAdd you can convert this date to the last date of month:

' Last Date of month: first add 1 month then subtract 1 day
DateAdd("d", -1, (DateAdd("m", 1, DateSerial([post-year], [post_period], 1)))

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First, use this expression to create your date as the first day of the period directly:

    DateSerial([post-year], [post_period], 1)

Then, add your form reference as a parameter in the query. Specify data type Date. This is the key to solve your issue:

    [Forms]![frmMiscellaneousTab]![frmMiscellaneousAcct]![EndingDate]

or as:

    [Forms]![frmMiscellaneousTab]![frmMiscellaneousAcct].Form![EndingDate]

/gustav

Thank you both for your replies.  I did not author this query, so I cannot claim to fully understand it why it was created as it is.   The goal is for the query to generate results where the ending date the user enters is less than or equal to the values in the columns post-period and post-year.    I understand this is trying to concatenate the post-period and post-year into a date format (CDate(Format(([post-period] & "  " & [post-year]),"mm/yyyy")))  - and I see the value in chaau's suggestion to make this:
(CDate(Format(([post-period] & "/" & [post-year]),"mm/yyyy")))

I created a new simple query to see what results I get from:

SELECT DateSerial([post-year],[post-period],1) AS SerialFunc, (CDate(Format(([post-period] & "/" & [post-year]),"mm/yyyy"))) AS CDateFunc

and they both produce the same results

That's good, but I don't understand how I use that to filter the results from the table if the user enters 8/31/14 as the ending date and it needs for find all the records where the post-period & post-date are less than or equal to 8/31/14,

why or how I would use DateAdd function or Gustav's comment to specify the data type of Date.

How about this:  

Ah I see Gustav - yes that does work!
Helen - I did initially try a query as you were suggesting but I ended up with an incorrect number of records.  My real query is using grouping and some other expressions that I didn't include in this ticket for simplicities sake.  I knew the issue was with the CDate() function.  

Thank you to all the experts that replied - I apprecriate your guidance.

I appreciate the explanation given!

You are welcome!

/gustav