Getting a running total of the number of rows after a given date but before another date?

[Anonymous]

So I have a table listing courses, with a "created_at" and an "expires_at" columns that have date values. I want to calculate the number of active courses for a long period, consisting of those courses that have been created but haven't been expired. I have a date table, simply called Dates and the Date column in Dates and the "created_at" column in courses are related. How do I calculate the number of courses offered on any given date?
This is what I wrote but it doesn't work. Returns a strange looking step graph for some reason.

Offered courses =

VAR Created = CALCULATE(

    COUNTA('courses course'[created_at]),

    FILTER(

        ALLSELECTED('Dates_early'[Date]),

        ISONORAFTER('Dates_early'[Date], MAX('Dates_early'[Date]), DESC)

    )

)

VAR Expired = CALCULATE(

    COUNTA('courses course'[expires_at]),

    FILTER(

        ALLSELECTED('Dates_early'[Date]),

        ISONORAFTER('Dates_early'[Date], MAX('Dates_early'[Date]), DESC)

    )

)

RETURN Created - Expired

Help?

[lbendlin]

Please provide sanitized sample data that fully covers your issue. If you paste the data into a table in your post or use one of the file services it will be easier to work with. Please show the expected outcome.