DateDiff Measure

[Anonymous]

Im having trouble showing the correct calculation of days between stages.

Shown below measure / formula by DAX code.

Firstly I need to create a calculate column [Index By Q No]  to set index for every group using DAX below:

Index By Q No = RANKX(FILTER(Table1, Table1[Q No] = EARLIER(Table1[Q No])), RANKX(ALL(Table1), Table1[Last Modified]), , DESC, Dense)

Then create a calcualte column to calculate datediff which is your expected result:

DateDiff = 
VAR Previous_Date = CALCULATE(MAX(Table1[Last Modified]), FILTER(Table1, Table1[Index By Q No] = EARLIER(Table1[Index By Q No]) - 1 && Table1[Q No] = EARLIER(Table1[Q No])))
RETURN 
DATEDIFF(Previous_Date, Table1[Last Modified], DAY)

The output is as below.

The index By Q No is not in sequence and the date diff measure shows wrong calculation.

May I know which steps I did wrong ? How do I get the accurate Index By Q No in sequence ?