Index column in power query based on date sorting

Hello Anonymous 

this code should do the trick

let
	Source = #table
	(
		{"ID","Sub ID","Date"},
		{
			{"ABC","ABC1","2020-01-01"},	{"ABC","ABC2","2020-01-02"},	{"ABC","ABC3","2020-01-03"},	{"ABC","ABC4","2020-01-04"},	{"DEF","DEF1","2020-01-02"},	{"DEF","DEF2","2020-01-05"},	
			{"DEF","DEF3","2020-01-04"},	{"DEF","DEF4","2020-01-03"}
		}
	),
	ToDate = Table.TransformColumns
	(
		Source,
		{
			{
				"Date",
				each Date.From(_,"en-US"),
				type date
			}
		}
	),
    Group = Table.Group(ToDate, {"ID"}, {{"AllRows", (tableint) => let sort = Table.Sort(tableint,{"Date", Order.Ascending}), AddIndex = Table.AddIndexColumn(sort,"Index",1) in AddIndex}}),
    Expand = Table.ExpandTableColumn(Group, "AllRows", {"Sub ID", "Date", "Index"}, {"Sub ID", "Date", "Index"})
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

If this post helps or solves your problem, please mark it as solution (to help other users find useful content and to acknowledge the work of users that helped you)
Kudoes are nice too

Have fun

Jimmy