Find days between different change types

There are likely a few ways to do this. Here is an example of one way...
start

end

code

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WCjAwVNJRckosziwG0gaG+kBkZGBkohSrA5N0LSrKLwJJmqBJGqHpNEaXdEwvSk3NTc0rASkwwqIAbrQxPkkzQkaboikwRnOYCbokusNQFJig6TZFl0R2NUwyFgA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Project ID" = _t, #"Change type" = _t, Date = _t]),
    Custom2 = Table.TransformColumns(Source, {{"Date", each Date.From(_, "de-DE"), type date}}),
    #"Changed Type" = Table.TransformColumnTypes(Custom2,{{"Project ID", type text}, {"Change type", type text}, {"Date", type date}}),
    Custom1 = Table.AddColumn(#"Changed Type", "lastNonErrorDate", each List.Max(Table.SelectRows(#"Changed Type", (x)=> x[Project ID] = [Project ID] and x[Change type] <> "Error" and x[Date] < [Date])[Date]), type date),
    #"Added Custom" = Table.AddColumn(Custom1, "Days", each if [Change type] = "Error" then Number.From([Date]) - Number.From([lastNonErrorDate]) else null, Int64.Type)
in
    #"Added Custom"

Hi RichArt ,

Please try this way:
First, duplicate the original table:

Put all of this M functions into the Advanced Editor in the duplicated table which will add an Index column for all rows where [Change type] = "Error":

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WCjAwVNJRckosziwG0ob6hvpGBkYmSrE6MCnXoqL8IrCUCbKUEZIuY1RdICnH9KLU1NzUvBKwtBG6NMxQY31j3FJm+A01RZY2RnKOCapzjNF0mqA6xwRJpymqThMk55jCXRoLAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [#"Project ID" = _t, #"Change type" = _t, Date = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Project ID", type text}, {"Change type", type text}, {"Date", type date}}),
    #"Filtered Rows" = Table.SelectRows(#"Changed Type", each ([Change type] = "Error")),
    #"Grouped Rows" = Table.Group(#"Filtered Rows", {"Project ID"}, {{"Count", each Table.AddIndexColumn(_, "Index", 1, 1, Int64.Type)}}),
    #"Expanded Count" = Table.ExpandTableColumn(#"Grouped Rows", "Count", {"Change type", "Date", "Index"}, {"Change type", "Date", "Index"})
in
    #"Expanded Count"

Then create a Blank Query:

Put all of these M functions into the Advanced Editor in the Blank Query:

let
    Source = Table.NestedJoin(Table, {"Project ID", "Change type", "Date"}, #"Table (2)", {"Project ID", "Change type", "Date"}, "Table (2)", JoinKind.LeftOuter),
    #"Expanded Table (2)" = Table.ExpandTableColumn(Source, "Table (2)", {"Index"}, {"Index"}),
    #"Added Custom" = Table.AddColumn(#"Expanded Table (2)", "Custom Date", each 
        let 
            currentRow = _,
            projectId = currentRow[Project ID],
            changeType = currentRow[Change type],
            index = currentRow[Index],
            date = currentRow[Date]
        in 
        if changeType = "Error" then 
            if index = 1 then 
                let 
                    sameProjectRows = Table.SelectRows(#"Expanded Table (2)", each [Project ID] = projectId),
                    filteredRows = Table.SelectRows(sameProjectRows, each [Change type] <> "Error" and [Date] < date),
                    maxDate = if 
                    Table.IsEmpty(filteredRows) then null else List.Max(Table.Column(filteredRows, "Date"))
                in 
                    maxDate 
            else 
                let 
                    sameProjectPreviousIndexRow = Table.SelectRows(#"Expanded Table (2)", each [Project ID] = projectId and [Change type] = "Error" and [Index] = index - 1),
                    previousDate = if Table.IsEmpty(sameProjectPreviousIndexRow) then null else sameProjectPreviousIndexRow{0}[Date]
                in 
                    previousDate
            else 
                null 
        ),
    #"Added Custom1" = Table.AddColumn(#"Added Custom", "Custom", each [Date] - [Custom Date]),
    #"Changed Type" = Table.TransformColumnTypes(#"Added Custom1",{{"Custom", type number}})
in
    #"Changed Type"

The final output is as below:

Best Regards,
Dino Tao
If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.

Hi dufoq3  and jgeddes 

Thank you both so much for these fast reply's and solutions.  I am really gratefull there are still people who are willingly and able to help others, thanks!

I set up multiple variations to double check and the options from the both of you are a solution and work great.

Actually i will stick to the solution of jgeddes  since I am able to keep the other columns as well, probarly the solution from dufoq3 would be able to do so but i cannot find how at this moment.  I did not implement it on my actual data set but will be doing this tonight, i will keep you posted.

Thank again!