Count of Days Between

Caitlin_Knox- OK, I believe I have it. I based this on my Quick Measure, Open Tickets here:

https://community.powerbi.com/t5/Quick-Measures-Gallery/Open-Tickets/m-p/409364

You do not want a relationship between your two tables and the formula you want is:

TotalDays = 
VAR __PTOTable = SELECTCOLUMNS(ScheduledPTO,"StartDate",[NextStartDate],"EndDate",[NextEndDate])
VAR __tmpTable = 
FILTER(
    GENERATE(
        __PTOTable,
        'Calendar'
    ),
    [Date]>=[StartDate] &&
    [Date]<=[EndDate]
)
RETURN COUNTROWS(__tmpTable)

BTW, I learned this technique reading @Phil_Seamark's insightful guidance and examples in his fantastic new book, Beginning DAX with Power BI: The SQL Pro’s Guide to Better Business Intelligence

Well worth the price of admission!

I also attached the PBIX file.