Sequence INDEX/EALIER DAX or Power Query

Index =
VAR __dt = DATA[Date]
VAR __tb =
    FILTER(
        DATA,
        DATA[Employee] = EARLIER( DATA[Employee] )
            && DATA[Date] < __dt
    )
VAR __streak =
    ADDCOLUMNS(
        __tb,
        "@contiguous",
            VAR __d = DATA[Date]
            VAR __prev =
                MAXX( FILTER( __tb, DATA[Date] < __d ), DATA[Date] )
            RETURN
                __prev + 1 = __d
    )
RETURN
    IF(
        MAXX( __tb, DATA[Date] ) + 1 = __dt,
        __dt - MAXX( FILTER( __streak, NOT [@contiguous] ), DATA[Date] ) + 1,
        1
    )

 

It's easier to handle in PQ,

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45W8srIz8urVNJRMtI31TcyMDJUitVBETXDKmqOVdQCq6glVlFDA+zChtiFsTvOELvrDLFbaYRkZUREBFDMWB9snRGKkBGmEMSxQLFYAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Employee = _t, Date = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Employee", type text}, {"Date", type date}}),

    Grouped = Table.Group(#"Changed Type", "Employee", {"grp", each let l=[Date], index = List.Accumulate({1..List.Count(l)-1}, {1}, (s,c) => if Duration.Days(l{c}-l{c-1})=1 then s&{List.Last(s)+1} else s&{1}) in Table.FromColumns(Table.ToColumns(_) & {index}, {"Empl", "Date", "Index"})}),

    #"Expanded grp" = Table.ExpandTableColumn(Grouped, "grp", {"Date", "Index"}, {"Date", "Index"})
in
    #"Expanded grp"