Day difference bettween dates (and matching ids)

Hi Anonymous,

I made one sample for your reference. Please check the steps as below. 

1. Enter the sample data and sort the date visited and id columns in power query.

2. Insert an index column, Plese refer to the M code as below.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("hY9LDsAgCETvwppEPtXSsxjvfw2ttga6acKGecMAtQIDghBbIh41GtrCmahAwwqyJUmkwTMqT496T8zRW4geXR4xP7WCDncQX0/QJNmT7AnHmxHYyss0rsC54POWLcKmv6x1", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type text) meta [Serialized.Text = true]) in type table [id = _t, #"date visited" = _t, #"days passed" = _t, #"next visit" = _t]),
    #"Changed Type1" = Table.TransformColumnTypes(Source,{{"date visited", type date}, {"next visit", type date}, {"days passed", type number}}),
    #"Changed Type" = Table.TransformColumnTypes(#"Changed Type1",{{"id", Int64.Type}, {"date visited", type text}, {"days passed", Int64.Type}, {"next visit", type date}}),
    #"Changed Type2" = Table.TransformColumnTypes(#"Changed Type",{{"date visited", type date}}),
    #"Sorted Rows" = Table.Sort(#"Changed Type2",{{"id", Order.Ascending}, {"date visited", Order.Ascending}}),
    #"Added Index" = Table.AddIndexColumn(#"Sorted Rows", "Index", 1, 1)
in
    #"Added Index"

3. Create the measures as below.

dayps = 
VAR minind =
    CALCULATE ( MIN ( Table1[Index] ), ALL ( Table1 ), VALUES ( Table1[id] ) )
VAR mindate =
    CALCULATE (
        MIN ( 'Table1'[date visited] ),
        ALL ( Table1 ),
        VALUES ( Table1[id] )
    )
VAR maxid =
    MAX ( Table1[id] )
VAR maxindex =
    MAX ( Table1[Index] )
VAR nextid =
    CALCULATE (
        MAX ( Table1[id] ),
        FILTER ( ALL ( Table1 ), Table1[Index] = maxindex - 1 )
    )
VAR maxdate =
    MAX ( 'Table1'[date visited] )
RETURN
    IF (
        minind = maxindex,
        0,
        IF ( nextid = maxid, DATEDIFF ( mindate, maxdate, DAY ) )
    )

next = 
VAR maxid =
    MAX ( Table1[id] )
VAR maxindex =
    MAX ( Table1[Index] )
VAR nextid =
    CALCULATE (
        MAX ( Table1[id] ),
        FILTER ( ALL ( Table1 ), Table1[Index] = maxindex + 1 )
    )
VAR result =
    CALCULATE (
        MAX ( Table1[date visited] ),
        FILTER ( ALL ( Table1 ), Table1[Index] = maxindex + 1 )
    )
RETURN
    IF (
        maxid = nextid
            && MAX ( Table1[date visited] ) <> result,
        result,
        BLANK ()
    )

For more details, please check the pbix as attached.

Regards,

Frank