Date difference by condition

You can Group by Customer and Manager, then extract the relevant data from each subgroup

In the code below, I used an Excel table as the data source, and had to do some trimming to remove the spaces from pasting your posted data, but you can use other data sources.

The below code is pasted into the Advanced Editor of Power Query, but the first four lines will need to be changed to reflect your actual data source.

let

//Change next four lines to reflect your actual data source
    Source = Excel.CurrentWorkbook(){[Name="Table11"]}[Content],
    #"Trim Column Names" = Table.RenameColumns(Source, List.Transform(Table.ColumnNames(Source), each {_, Text.Trim(_)})),
    #"Changed Type" = Table.TransformColumnTypes(#"Trim Column Names",{
        {"Date of order", type date }, {"Customer", type text}, {"Manager", type text}}, "en-GB"),
    #"Trim Data" = Table.TransformColumns(#"Changed Type", {
        {"Customer", Text.Trim},
        {"Manager",Text.Trim}}),

//Group by Customer and manager
//Then extract, from each subgroup, the relevant data
    #"Grouped Rows" = Table.Group(#"Trim Data", {"Customer", "Manager"}, {
        {"Recent Orders and Difference", (t)=>
            let
                Orders = List.MaxN(t[Date of order],2),

            //ensure there are two entries for each subgroup, even if only one date
                #"Last 2 Orders" = if List.Count(Orders) = 1 then Orders & {null} else Orders,
                
                diff = Duration.Days(#"Last 2 Orders"{0} - #"Last 2 Orders"{1}),
                #"As Records" = Record.FromList(#"Last 2 Orders" & {diff},{"last order", "Previous order", "Date difference"}) 
            in 
                #"As Records", type record}
        }),
    #"Expanded Recent Orders and Difference" = Table.ExpandRecordColumn(#"Grouped Rows", "Recent Orders and Difference", {"last order", "Previous order", "Date difference"}),
    #"Changed Type1" = Table.TransformColumnTypes(#"Expanded Recent Orders and Difference",{{"last order", type date}, {"Previous order", type date}, {"Date difference", Int64.Type}})
in
    #"Changed Type1"

. Results from your data

Thanks a lot, sir. Yeah, that's what I'm trying to do (about grouping by parameters). On SQL it's just a row "group by table11.customer, table11.manager", but on M I didn't know the correct functions.

Thx again!