Power Query Recursion: Simple reference of same row

Maybe this is what you are looking for. Just a note, though, that recursions usually perform very badly in Power Query and there is usually a way more performant, non-recursive method to get the same results. This is partly why lots of people are giving you non-recursive solutions.

SourceData:

let
    Source = SourceData,
    
    AddPreviousHigher = 
    let 
        srcBuf = Table.Buffer( Source )
    in 
        Table.AddColumn(
            srcBuf, 
            "PreviousHigherDate", 
            each let 
                recF = ( x as number ) => 
                let thisDate =  srcBuf{x}[Date] in
                if x = 0 
                    then thisDate 
                    else let prevDate = srcBuf{x-1}[Date] in 
                        if thisDate > prevDate
                            then thisDate 
                            else @recF( x - 1) //recursion
            in 
                recF([Index]),
            type date
        )
in
    AddPreviousHigher

Output:

Edit: realized I got the recursive logic wrong, tweaked to reflect OP's clarificaiton