Pivoting values with a hook

Question

Hi Folks
I want to generate a table that calculates the dates on which customers should have been billed (see Table 1). Customers get billed on the same day each month.  I have the date that customers were acquired and if applicable cancelled (Table 2). If a customer
cancels they are charged for the month.
 If there is no cancellation date, the customer is still active. New customers are acquired each month.
There is native pivoting functionality in Power Query. But the hook I am struggling with is to dynamically generate  the  dates between activation and cancellation or the current date.   
Any  suggestion  as to how this can be done?
I have attached a sample file  in case that helps.
Cheers
Steve
Table 1 Output
Table 2 Raw starting table

Answer 1

Hi Steve,
There are perhaps a few ways to do this. One method is to first calculate the number of months between the activation date, and current date or cancellation date as the case may be.
Next, for each entry in the table, generate a list that starts at 0 and ends at the number of months calculated. Each  entry in the expanded list will serve as the number of months to add to the activation date, which will be subsequently used
to generate the billing dates.
Power Query doesn't have a DateDifference function like Excel, so I created a MonthDiffBetweenDates custom function:
(startDate as date, endDate as date) as any =>
(Date.Year(endDate)-Date.Year(startDate))*12+Date.Month(endDate)-Date.Month(startDate)
The main query was generated entirely through the UI. The expressions in the inserted columns were done by hand, through the UI (the Table1 source is your "Table 2 Raw starting table"):
let
    Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content], 
    ChangedType = Table.TransformColumnTypes(Source,{{"Activation Date", type date}, {"Cancellation Date", type date}}),
    InsertedCustom = Table.AddColumn(ChangedType, "Current Date", each Date.From(DateTime.LocalNow())),
    InsertedCustom1 = Table.AddColumn(InsertedCustom, "Months Elapsed", each if [Cancellation Date]=null then MonthDiffBetweenDates([Activation Date],[Current Date]) else MonthDiffBetweenDates([Activation Date],[Cancellation Date])),
    InsertedCustom2 = Table.AddColumn(InsertedCustom1, "Months to Add", each {0..[Months Elapsed]}),
    #"Expand Months to Add" = Table.ExpandListColumn(InsertedCustom2, "Months to Add"),
    InsertedCustom3 = Table.AddColumn(#"Expand Months to Add", "Billing Date", each Date.AddMonths([Activation Date],[Months to Add])),
    RemovedOtherColumns = Table.SelectColumns(InsertedCustom3,{"Subscriber ID", "Plan", "Billing Date"}),
    ReorderedColumns = Table.ReorderColumns(RemovedOtherColumns,{"Subscriber ID", "Billing Date", "Plan"}),
    ChangedType1 = Table.TransformColumnTypes(ReorderedColumns,{{"Billing Date", type date}})
in
    ChangedType1
There are four custom columns. The first custom column just gets the current date. The second custom column calculates the months elapsed from the activation date to current date (or cancellation date, if one exists). The third custom column is a months
to add list (expanded to individual rows). The third custom column simply uses the Date.AddMonths function to add months to the activation date. The rest of the code is cleanup.
Note that in the last step that I explicitly changed the Billing Date to type date, even though the query editor listed the column as having a date type and the dates appear as and perform like...well dates. If you don't perform this step, after
loading to the worksheet the dates in the date column appear as date serial numbers, instead of a formatted date. I have no idea why PQ behaves in this manner.
Hope this helps.