--  DATESINPERIOD returns an entire period, given a reference date and
--  the period.

--  The first query returns 1 day, starting from August 15, 2008
EVALUATE
    DATESINPERIOD ( 
        'Date'[Date],           -- Return dates in Date[Date]
        DATE ( 2008, 08, 15 ),  -- Starting from 08/15/2008
        1,                      -- the set needs to contain 1
        DAY                     -- day
    )
    
--  The second query returns 3 days, starting from August 15, 2008
EVALUATE
    DATESINPERIOD ( 
        'Date'[Date],           -- Return dates in Date[Date]
        DATE ( 2008, 08, 15 ),  -- Starting from 08/15/2008
        3,                      -- the set needs to contain 3
        DAY                     -- days
    )

--  The second query returns an empty table because we reques 0 days
EVALUATE
    DATESINPERIOD ( 
        'Date'[Date],           -- Return dates in Date[Date]
        DATE ( 2008, 08, 15 ),  -- Starting from 08/15/2008
        0,                      -- the set needs to contain 0
        DAY                     -- days
    )

--  When the offset is negative, DATESINPERIOD goes back to find
--  the dates to use

--  The first query returns 2 days, the last one is August 15, 2008
EVALUATE
    DATESINPERIOD ( 
        'Date'[Date],           -- Return dates in Date[Date]
        DATE ( 2008, 08, 15 ),  -- Starting from 08/15/2008
        -2,                     -- the set needs to contain 2
        DAY                     -- days, going back in time
    )
    
--  The second query returns an entire month (31 days), the last day is August 15, 2008.
--  The number of days for a month might vary between 28 and 31, depending on the month.
EVALUATE
    DATESINPERIOD ( 
        'Date'[Date],           -- Return dates in Date[Date]
        DATE ( 2008, 08, 15 ),  -- Starting from 08/15/2008
        -1,                     -- going back one
        MONTH                   -- month
    )

--
--  In this example we compute the moving annual average of [Sales Amount]
--  only when DATESINPERIOD contains 12 months of sales
--
DEFINE
    MEASURE Sales[Sales MAT] =
        VAR OneYearBack =
            DATESINPERIOD ( 'Date'[Date], MAX ( 'Date'[Date] ), -1, YEAR )
        VAR Result =
            CALCULATE (
                VAR NumberOfMonths =
                    COUNTROWS ( SUMMARIZE ( Sales, 'Date'[Calendar Year Month Number] ) )
                VAR SalesMAT = [Sales Amount]
                VAR Result   = IF ( NumberOfMonths = 12, SalesMAT / NumberOfMonths )
                RETURN
                    Result,
                OneYearBack
            )
        RETURN
            Result
EVALUATE
SUMMARIZECOLUMNS (
    'Date'[Calendar Year Month Number],
    'Date'[Calendar Year Month],
    TREATAS ( { "CY 2008" }, 'Date'[Calendar Year] ),
    "Sales Amount", [Sales Amount],
    "Sales MAT", [Sales MAT]
)
ORDER BY [Calendar Year Month Number]