Expression to display measure for previous month only

[RoyBatty]

Hi,

We have the following table:

messages:
LOAD
    country,
    message_count,
    client_cost,
    sent_month
FROM [lib://QVDFOLDER/messages.qvd] (qvd);

sent_month is a Date in format MMM YYYY (e.g. Jun 2025) :

and Data model viewer shows the following tags:

On the sheet we added a table, and as the first column (dimension) we added country.

Now we want to add a new column (measure) that will display Sum([message_count]) for the previous month only. 

We tried using the following expression:
Sum({<[sent_month] = {"=$(Date(MonthStart(AddMonths(Today(), -1)), 'MMM YYYY'))"}>} message_count)

But it doesn't work:

We checked the data and there are messages that were sent in the previous month, but when we use the above expression - it shows an empty table. 

We tested this part of the expression by adding it to be a label - and it works:

Date(MonthStart(AddMonths(Today(), -1)), 'MMM YYYY')

Do you know why the above expression doesn't work, what might need to be changed to make it work?

[Digvijay_Singh]

Try this may be - 

Sum({<[sent_month] = {"=$(=Date(MonthStart(AddMonths(Today(), -1)), 'MMM YYYY'))"}>} message_count)

Check in the expression editor at the bottom, it should show right value of the expression used in sent_month.

Somehow formatting should match between data model value and the value resulting from the above expression. Check for decimal component in the date as well, sometimes that causes the mismatch.

[RoyBatty]

I tried that  and now the editor shows June 2025 (please ignore the "msg." prefix - I've excluded it here for simplicity):

but for some reason it still doesn't work:

[RoyBatty · Accepted solution]

This works (the problem was with the quotes): 

Sum({<[sent_month] = {"$(=Date(MonthStart(AddMonths(Today(), -1)), 'MMM YYYY'))"}>} message_count)

(and also with missing "="... Thanks @Digvijay_Singh !)