P4

 let

    // Assuming "Table" is the name of your table and "Date" is the name of your date column

    // Find the previous date with a non-null value

    FillDates = (currentDate) =>

        let

            previousDate = List.LastN(Table.SelectRows(Table, each [Date] < currentDate and [Date] <> null), 1),

            result = if List.Count(previousDate) = 0 then currentDate else Record.Field(previousDate{0}, "Date")

        in

            result,

    // Fill null values based on the previous day's value

    CustomColumn = Table.AddColumn(Table, "FilledDate", each FillDates([Date])

in

  

  CustomColumn