Calculate Last Twelve Months Using DAX

One of the more common calculations a company uses is the last twelve months, or LTM, of data. This can be tricky if your date table always has a full year of dates for the current year, which it generally should. So if today is March 3, 2019, my date table will have dates through December 31, 2019. This is usually necessary for the date intelligence functions in DAX to work properly, and companies may have data beyond today in their model. For example, budget and forecast data will generally extend through the end of the year, or at least beyond today.

However, it often is challenging when you are trying to hide these future dates for specific measures. I’ve seen solutions that use functions like LASTNONBLANK() that get the last date with sales data in it, and that can work, but depending on how your data is laid out, it can make for larger and more complex measures with multiple FILTER() functions. For a visual you can sometimes use the relative filtering feature, but that won’t change the underlying value of the measure if you reuse it in another visual or refer to it from another measure.

Marco Russo recently wrote an excellent post on hiding future dates or calculations in DAX. The concept is brilliantly simple. Just add a column to your date table that returns TRUE if the date is today or earlier, or FALSE if it is after today, then use the CALCULATETABLE() function to return just a table of dates that fall in that TRUE range of dates.

That wouldn’t work for me though exactly as it was presented. I needed to create dates that were in the previous 12 calendar months, and I was working with a Power BI Dataset, which is a Live Query, and you cannot add columns to Live Query models.

So I opted to create two measures. First, I needed to create the date logic in my dates table. I wanted the previous 12 full calendar months, not the last 365 days of data. Note that my date table is named ‘Calendar’.

LTM Dates = 
VAR EndDate =
    EOMONTH ( TODAY (), -1 )
VAR StartDate =
    EDATE ( EOMONTH ( TODAY (), -1 ), -12 ) + 1
RETURN
    IF (
        MAX ( 'Calendar'[Date] ) >= StartDate
            && MAX ( 'Calendar'[Date] ) <= EndDate,
        TRUE (),
        FALSE ()
    )

This measure has two variables:

1. EndDate - This calculates the last day of the month for the previous month based on TODAY().

2. StartDate - This calculates the month 12 months prior to the EndDate, then adds one day to move to the first day of the next month.

Finally the measure uses a basic IF() statement, with some AND logic. If today is March 3, 2019, it will return TRUE for the dates March 1, 2018 through February 28, 2019. For dates before March 1, 2018, and after February 28, 2019, it returns FALSE. It will do the for the entire month of March. On April 1, the LTM range becomes April 2018 - March 2019.

I could have used the AND() function instead of the double ampersand, but I use the double ampersand as I can use multiple conditions, like condition1 && condition2 && condition3, whereas AND() is limited to two conditions. By getting in the habit of using &&, I never have to remove an AND() function and redo the syntax. Side note: Use double pipes to allow multiple conditions for OR logic. Condition1 || condition2 || condition3, as OR() is also restricted to two conditions.

Now I needed to calculate sales for LTM. I already had the [Total Sales] measure below:

Total Sales = 
CALCULATE(
    SUM(Sales[Sales]),
    Sales[Type]="ACT"
) 

The measure for [Sales LTM] then is:

Sales LTM = 
CALCULATE(
    [Total Sales],
    CALCULATETABLE(
        'Calendar',
        FILTER(
            'Calendar',
            [LTM Dates] = TRUE()
        )
    )
)

You could combine my first measure with the second measure, replacing [LTM Dates] with the full measure, after tweaking the date logic in the FILTER() section a bit in this [Sales LTM2] measure.

Sales LTM2 =
VAR EndDate =
    EOMONTH ( TODAY (), -1 )
VAR StartDate =
    EDATE ( EOMONTH ( TODAY (), -1 ), -12 ) + 1
RETURN
    CALCULATE (
        [Total Sales],
        CALCULATETABLE (
            'Calendar',
            FILTER (
                'Calendar',
                'Calendar'[Date] >= StartDate
                    && 'Calendar'[Date] <= EndDate
            )
        )
    )

However, this measure is both a bit more complex, and if you wanted to have other LTM measures, such as units sold, or cost of goods over the last year, dollars purchased, etc., you’d have to repeat the date logic in each measure. If you wanted to change the LTM logic, say switch from previous 12 completed calendar months to last 365 days, or last 12 calendar months but starting with this month, you’d have to edit every measure calculating the date range. By breaking it into two parts as I’ve done above, I can edit just the [LTM Dates] measure and all other measures that use it will automatically recalculate accordingly.

Also note that unlike Marco’s solution, my date measure will not behave as a calculated column.

1. You could not use it in an iterator function such as SUMX(), AVERAGEX(), and so on, as iterators use row context, and measures generally do not have row context. Well, iterator measures do, but they have to have row context to start with. They cannot create it out of thin air.

2. You also cannot use measures in slicers or filters in your report. For those, you must use either a calculated column, or bring the column in through Power Query.

3. You cannot use it as the date column in a date intelligence function, because it isn’t a column.

Calculated columns, and better yet imported columns via Power Query, can be a better choice for the above secenario, but that is not always an option if your source data is from SSAS or a Power BI Dataset where adding columns isn’t permitted.