Creating Date Tables

IMHO, the fastest way is:

1. Open a blank query in Power Query of Power BI
2. type ={Number.From(#date(2018,1,1))..Number.From(#date(2018,12,31))}
3. That will generate a series of numbers as a list
4. Convert it to a table (upper left menu button.
5. Convert the ABC123 type to date
6. Rename to Date.
7. Now you have a date table. Add columns as necessary (year, month, month name, etc) to make your date table suit your needs.
8. Close and load.
9. Right-click on it and mark it as a date table.

If you have source data with dates in it, get fancy and find the earliest date in your data, then make row #2 above be Jan 1, YYYY where YYYY is the earliest date in your dataset,

So if your original line is like this, you just need to use some functions to determine the dates vs hardcoding.

={Number.From(#date(2018,1,1))..Number.From(#date(2018,12,31))}

This will always give you a rolling 6 months. I've inserted a lot of line feeds to make the formulas a bit easier to read, but you could type that Source line all on one line. The key to all of this is DateTime.LocalNow() - that is equivalent to @NOW() in Excel - the current date and time from the system clock.

let
    Source = 
        {
            Number.From(
                Date.AddMonths(
                    DateTime.Date(
                        DateTime.LocalNow()
                    ),
                    -6
                )
            )..
            Number.From(
                DateTime.Date(
                    DateTime.LocalNow()
                )
            )          
        },
    #"Converted to Table" = Table.FromList(Source, Splitter.SplitByNothing(), null, null, ExtraValues.Error),
    #"Changed Type" = Table.TransformColumnTypes(#"Converted to Table",{{"Column1", type date}})
in
    #"Changed Type"