Distributing Hours over Months using START/END dates/time.

I think it's because you are adding this as a custom column. Not as a function like I told you.

From New Source select Blank Query. Name this query on the left box (like changing the name of the source table). Then open Advanced Editor (it's a button next to the Refresh Preview)and paste this:

let getParameters = (StartDate, EndDate) =>
    
    let

        NullStart = StartDate = "" or StartDate = null,
        Start = if NullStart then null else Date.From(StartDate),
        NullEnd = EndDate = "" or EndDate = null,
        End = if NullEnd then null else Date.From(EndDate),

        CountDays = if NullEnd or NullStart then 1 else Duration.Days(End-Start) + 1,

        DateList = if NullStart then {null} else List.Dates(Start, CountDays, #duration(1, 0, 0, 0)),

        #"Converted to Table" = Table.FromList(DateList, Splitter.SplitByNothing(), {"Date"}, null, ExtraValues.Error),
        #"Add Custom Start DateTime" = Table.AddColumn(#"Converted to Table", "Start datetime", each if NullStart then null else if [Date] = Start then StartDate else DateTime.From([Date])),
        #"Add Custom End DateTime" = Table.AddColumn(#"Add Custom Start DateTime", "End datetime", each if NullEnd then null else if [Date] = End then EndDate else [Date] & #time(23,59,59)),
        #"Change Types" = Table.TransformColumnTypes(#"Add Custom End DateTime",{{"End datetime", type datetime}, {"Start datetime", type datetime}}),
        #"Duration" = Table.AddColumn(#"Change Types", "Duration", each Duration.TotalMinutes([End datetime]-[Start datetime])/60, type number),
        #"Remove Date" = Table.RemoveColumns(#"Duration",{"Date"})
        
    in
        #"Remove Date"
        
in
    getParameters

After doing that you should see something like this:

Then go to your table and from ribbon select new column > invoke custom function:

Select column name, query name that you have provided in previous steps and the start and end columns (example on screenshot).

And you should know the rest. 🙂