Need help with DMR

[Wild Klaus]

Hello guys.
Please help with the creation of the calculation.
There are dimension "Date of operation" and indicator of "Number".
Need to calculate the difference in the first (not always 1) and the last number (not always 29-30-31) months.
I can not get the first and last days.
I am using the formula:
First_day: item(tail(members([Report].[Operation_date].[Calc_period].[Day])),0)
Last_day: item(head(members([Report].[Operation_date].[Calc_period].[Day])),0)
But works only some one :(
I am adding First_day and see the data in report, but when I add the Last_day, the information is not available.
I am adding Last_day and see the data in report, but when I add the First_day, the information is not available.