Add integer hours to date in Power Query or with DAX

Based on your description, I created data to reproduce your scenario. The pbix file is attached in the end.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45W8srPyFPSUTK00DMw1zMyMDIBcpycnICksVKsDnZ5R0dHIGmKIm+JJO/s7AwkLZDljQzwm48iD9GPWx5ivxFY3juxKBPDgcgOgCuwxDDBBLcCiBOBCmIB", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Name = _t, Date = _t, Task = _t, Hours = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Name", type text}, {"Date", type date}, {"Task", type text}, {"Hours", Int64.Type}}),
    #"Grouped Rows" = Table.Group(#"Changed Type", {"Name", "Date"}, {{"Count", 
    (x)=>[a =Table.ToRecords(x),
     b =List.Generate (
    () => [ i = 0, result = a{i}[Date]&#time(9+a{i}[Hours],0,0) ], 
    ( x ) => x[i] < List.Count ( a ), 
    ( x ) => [ i = x[i] + 1, result = ( x[result] +#duration(0, a{i}[Hours],0,0) )  ], 
    ( x ) => Record.AddField ( a{x[i]}, "End DateTime", x[result] )
  ),
  c =  List.Transform(b,(x)=>  Record.AddField ( x,"Start DateTime",x[End DateTime]-#duration(0,x[Hours],0,0)  )),
  d = Table.FromRecords ( c )
   
  ][d]}}),
    #"Removed Other Columns" = Table.Combine( Table.SelectColumns(#"Grouped Rows",{"Count"})[Count]),
    #"Changed Type1" = Table.TransformColumnTypes(#"Removed Other Columns",{{"End DateTime", type datetime}, {"Start DateTime", type datetime}}),
    #"Reordered Columns" = Table.ReorderColumns(#"Changed Type1",{"Name", "Date", "Task", "Hours", "Start DateTime", "End DateTime"}),
    #"Removed Columns" = Table.RemoveColumns(#"Reordered Columns",{"Date"})
in
    #"Removed Columns"

Hi mrbartuss, another solution (List.Accumulate):

Result:

let
    fnDateTimes = 
        (myTable as table)=>
        [
            // _Detail = GroupedRows{2}[All],
            _Detail = myTable,
            _BufferedHours = List.Buffer(_Detail[Hours]),
            _AccumulatedDateTimes = List.Accumulate(
                {1..List.Count(_BufferedHours)-1},
                {{ _Detail{0}[Date] & #time(9,0,0), _Detail{0}[Date] & #time(9,0,0) + #duration(0,_BufferedHours{0},0,0) }},
                (s,c)=> s & {{ List.Last(s){1}, List.Last(s){1} + #duration(0,_BufferedHours{c},0,0) }}
            ),
            _RemovedColumns = Table.RemoveColumns(_Detail, {"Hours"}),
            _ToTable = Table.FromColumns(Table.ToColumns(_RemovedColumns) & Table.ToColumns(Table.FromRows(_AccumulatedDateTimes)), Value.Type(_RemovedColumns & #table(type table[Start DateTime=datetime, End DateTime=datetime], {})))
        ][_ToTable],

    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45W8srPyFPSUTK00Dcw1zcyMDIBcpycnICksVKsDnZ5R0dHIGmKIm+JJO/s7AwkLZDljQzwm48iD9GPWx5ivxFY3juxKBPDgcgOgCuwxDDBBLcCiBOBCmIB", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Name = _t, Date = _t, Task = _t, Hours = _t]),
    ChangedType = Table.TransformColumnTypes(Source,{{"Hours", type number}, {"Date", type date}}, "sk-SK"),
    GroupedRows = Table.Group(ChangedType, {"Name", "Date"}, {{"fn", fnDateTimes, type table}}),
    Combined = Table.Combine(GroupedRows[fn])
in
    Combined

Ahmedx dufoq3 Thank you both very much!
Below is my solution. How would you rate all three solutions based on performance?

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45W8srPyFPSUTK00DMw1zMyMDIBcpycnICksVKsDnZ5R0dHIGmKIm+JJO/s7AwkLZDljQzwm48iD9GPWx5ivxFY3juxKBPDgcgOgCuwxDDBBLcCiBOBCmIB", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Name = _t, Date = _t, Task = _t, Hours = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Name", type text}, {"Date", type datetime}, {"Task", type text}, {"Hours", Int64.Type}}),
    Group = Table.Group(
    #"Changed Type",
    {"Name", "Date"},
    {{"All", each Table.AddIndexColumn(_, "Index", 1, 1, Int64.Type), type table}}
  ),
  Result = Table.AddColumn(Group, "Result", each let
        BuffValues = List.Buffer([All][Hours]),
        RunningTotal = List.Generate(
 
          // Start value: Set RT to 1st value in BuffValues and set RowIndex to 0
          () => [RT = BuffValues{0}, RowIndex = 0],
          // Condition: Generate values as long as RowIndex is < than number of list items
          each [RowIndex] < List.Count(BuffValues),
          // Record with logic for next values:
          each [
 
            // Calculate running total by adding current RT value and the next value
            RT = List.Sum({[RT], BuffValues{[RowIndex] + 1}}),
            // Increment the RowIndex by 1 for the next iteration
            RowIndex = [RowIndex] + 1
          ],
          // Only return the running total (RT) value for each row
          each [RT]
        ),
        CombinedTable = Table.FromColumns(
          Table.ToColumns([All]) & {Value.ReplaceType(RunningTotal, type {Int64.Type})},
          Table.ColumnNames([All]) & {"Running Total"}
        ),
        StartDateStage = Table.AddColumn(
          CombinedTable,
          "Start Date Stage",
          each [Date] + #duration(0, 9, 0, 0),
          type datetime
        ),
        EndDate = Table.AddColumn(
          StartDateStage,
          "End Date",
          each [Start Date Stage] + #duration(0, [#"Running Total"], 0, 0),
          type datetime
        ),
        StartDate = Table.AddColumn(
          EndDate,
          "Start Date",
          each if [Index] = 1 then [Start Date Stage] else EndDate[End Date]{[Index] - 2},
          type datetime
        ),
        Result = Table.SelectColumns(
          StartDate,
          {
            "Name",
            "Date",
            "Hours",
            "Task",
            "Start Date",
            "End Date"
          }
        )
      in
        Result),
  FinalResult = Table.Combine(Table.SelectColumns(Result, {"Result"})[Result])
in
    FinalResult

orks a little slow

here is the difference in time