Can't add a Timeline

gotAnAccount 

That's hard to say exactly without seeing the data, but I suspect you don't have real data in final result. To convert left 3 columns to date

it could be code like

    #"Replaced text null to value null" = Table.ReplaceValue(
        #"Promoted Headers",
        "null",
        null,
        Replacer.ReplaceValue,{"year"}
    ),
    #"Added Custom" = Table.AddColumn(
        #"Replaced text null to value null",
        "Date",
        each
            if Text.Length([year]) = 4
            then 
                #date(
                    Number.From([year]),
                    Number.From([month]),
                    Number.From([day])
                )
            else [year]
    ),
    #"Replaced Value" = Table.ReplaceValue(
        #"Added Custom",
        null,
        #date(9999, 1, 1),
        Replacer.ReplaceValue,{"Date"}
    ),
    #"Apply Date Type" = Table.TransformColumnTypes(
        #"Replaced Value",
        {
            {"Date", type date}
        }
    )
in
    #"Apply Date Type"

Having mixed of texts, dates and numbers in one column it's easy to make error in transformation. After you return transformed data to Excel and create PivotTable it's also could be loss of formatting. But that easy to check with =ISTEXT()