What problem?

Please be more specific. See Rule #2.

sorry i do not know why it appeared in a new post, i meant this problem
- i watched the Month over month lesson and i followed all the steps but i have difficulty to understand one point. if the comparison is between months in the same year, the formula shows the sum of sales for 2 months instead of showing only the previous month.

should not be the sales of Feb. 2021 600 under the column of PrevMonthSales instead of 900 when i compare it with mar 2021?

I just did some testing and this is a date problem. To get the correct values Format the dates to the US date format. For example: Format([BeginDate],"mm/dd/yyyy")

Thank you Kevin