Find days between different change types

Hi RichArt,

Solution 1 (if you don't want date sort)

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]),
    ChangedType = Table.TransformColumnTypes(Source,{{"Date", type date}}, "sk-SK"),
    GroupedRows = Table.Group(ChangedType, {"Project ID"}, {{"All", each Table.Sort(_, {{"Date", Order.Ascending}}) , type table}}),
    Ad_ErrorPositions = Table.AddColumn(GroupedRows, "ErrorPositions", each List.PositionOf([All][Change type], "Error", Occurrence.All), type list),
    Ad_Days = Table.AddColumn(Ad_ErrorPositions, "Days", each
     [ a = { Duration.TotalDays([All][Date]{[ErrorPositions]{0}} - [All][Date]{[ErrorPositions]{0}-1}) },
       b = if List.Count([ErrorPositions]) > 1 then a & List.Transform(List.Skip(List.Zip({ [ErrorPositions], {null} & List.RemoveLastN([ErrorPositions], 1) })), (x)=> Duration.TotalDays([All][Date]{x{0}} - [All][Date]{x{1}}))
           else if List.Count([ErrorPositions]) = 1 then a else null
     ][b], type list),
    FiteredRows = Table.SelectRows(Ad_Days, each [Days] <> null),
    RemovedColumns = Table.RemoveColumns(FiteredRows,{"All", "ErrorPositions"}),
    ExpandedDays = Table.ExpandListColumn(RemovedColumns, "Days")
in
    ExpandedDays

Solution 2 (with date sort)

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]),
    ChangedType = Table.TransformColumnTypes(Source,{{"Date", type date}}, "sk-SK"),
    GroupedRows = Table.Group(ChangedType, {"Project ID"}, {{"All", each Table.Sort(_, {{"Date", Order.Ascending}}), type table}}),
    Ad_ErrorPositions = Table.AddColumn(GroupedRows, "ErrorPositions", each List.PositionOf([All][Change type], "Error", Occurrence.All), type list),
    Ad_Days = Table.AddColumn(Ad_ErrorPositions, "Days", each
       List.Transform([ErrorPositions], (x)=> Duration.TotalDays([All][Date]{x} - [All][Date]{x-1}))
       
  , type list),
    FiteredRows = Table.SelectRows(Ad_Days, each List.Count([Days]) > 0),
    RemovedColumns = Table.RemoveColumns(FiteredRows,{"All", "ErrorPositions"}),
    ExpandedDays = Table.ExpandListColumn(RemovedColumns, "Days")
in
    ExpandedDays

Here is my corrected code. dufoq3  solution 2 will run quicker than this on larger datasets.

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),
    #"Filtered Rows" = Table.SelectRows(#"Added Custom", each ([Days] <> null)),
    #"Removed Other Columns" = Table.SelectColumns(#"Filtered Rows",{"Project ID", "Days"})
in
    #"Removed Other Columns"