How to do MTD

Hi Supipa 

Here's a measure for you to try:

Bets Last Month = 
VAR _Today = TODAY()
VAR _LastDayThisMonth = EOMONTH(_Today, 0)
VAR _FilteredMonth = MAX('Calendar'[Date])
VAR _Result = 
IF(
    _LastDayThisMonth = _FilteredMonth,
    // get last month up to today's day of month
    CALCULATE(
        [Bets], 
        DATEADD(
            FILTER(VALUES('Calendar'[Date]), DAY('Calendar'[Date]) <= DAY(_Today)), 
            -1, 
            MONTH
        )
    ),
    // get all of last month
    CALCULATE ([Bets], PARALLELPERIOD('Calendar'[Date], -1, MONTH))
)
RETURN
    _Result

As you can see it's based on today's date.  I put it as a variable in case you wanted to use the latest date in the dataset instead.  In that case you'd put this instead

VAR _Today = CALCULATE(MAX(Bets[Date]), REMOVEFILTERS())