Rankby help - I think its RANKX!

Hello tanyad 

but this is how Power Query works. You have to apply a Table.Group and then Sort and add a Index-column.

Here the complete solution

let
	Source = #table
	(
		{"CustID","Code","Date"},
		{
			{"1000","A","43101"},	{"1000","B","42736"},	{"1000","B","43466"},	{"2000","A","43101"},	{"2000","A","42736"},	{"2000","B","43466"},	{"2000","B","43101"}

					}
	),
    ToDate = Table.TransformColumns
	(
		Source,
		{
			{
				"Date",
				each Date.From(Number.From(_)),
				type date
			}
		}
	),
    Group = Table.Group(ToDate, {"CustID", "Code"}, {{"AllRows", each _, type table [CustID=text, Code=text, Date=date]}}),
	SortAndIndex = Table.TransformColumns
	(
		Group,
		{
			{
				"AllRows",
				(Tableint)=> let 
				Sort = Table.Sort(Tableint, {{"Date", Order.Ascending}}),
				AddIndex = Table.AddIndexColumn(Sort, "Order",1,1)
				in 
				AddIndex
			}
		}
	),
    DeleteOther = Table.SelectColumns(SortAndIndex,{"AllRows"}),
    Expand = Table.ExpandTableColumn(DeleteOther, "AllRows", {"CustID", "Code", "Date", "Order"}, {"CustID", "Code", "Date", "Order"})
in
	Expand

Copy paste this code to the advanced editor in a new blank query to see how the solution works. If this solution fits your need, copy and past a part of it and implement it in your query.

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Kudoes are nice too

Have fun

Jimmy