Calculate the difference between each date

Juju123 See my article on Mean Time Between Failure (MTBF) which uses EARLIER: http://community.powerbi.com/t5/Community-Blog/Mean-Time-Between-Failure-MTBF-and-Power-BI/ba-p/339586.
The basic pattern is:
Column = 
  VAR __Current = [Value]
  VAR __PreviousDate = MAXX(FILTER('Table','Table'[Date] < EARLIER('Table'[Date])),[Date])

  VAR __Previous = MAXX(FILTER('Table',[Date]=__PreviousDate),[Value])
RETURN
  __Current - __Previous

Hi Greg_Deckler , 

thanks for your answer. 
i read your post about MBT and I take your code. 

i would like understand , what does [value] correspond to ?

i modify the code like this : 

Time each operation = 
  VAR _Current = [Value] ?
  VAR _PreviousDate = MAXX(FILTER('T1','Table'[Operation date] < EARLIER('T1'[Operation date])),[Operation date])

  VAR _Previous = MAXX(FILTER('T1',[Operation date]= VAR _PreviousDate),[Value])
RETURN
VAR _Current  - VAR _Previous

thanks you 

Hi, Juju123 

You can try the following methods.
Column:

Previous date = MAXX(FILTER('Table',[Date operation]<EARLIER('Table'[Date operation])),[Date operation])

Result = IF([Previous date]=BLANK(),0,[Date operation]-[Previous date])

Is this the result you expect?

Best Regards,

Community Support Team _Charlotte

If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.

v-zhangti, 

I added a column which is the operation number.

I modified this formula to retrieve the previous date for each Workflow/Article pair:

Previous_Date = MAXX(FILTER(ALL('CAGC_ADVL_INDICATEURS_ETAPES_WF_REP'),CAGC_ADVL_INDICATEURS_ETAPES_WF_REP[Numero de l'operation]<SELECTEDVALUE('CAGC_ADVL_INDICATEURS_ETAPES_WF_REP'[Numero de l'operation])&&[Designation article]=SELECTEDVALUE('CAGC_ADVL_INDICATEURS_ETAPES_WF_REP'[Designation article])),CAGC_ADVL_INDICATEURS_ETAPES_WF_REP[WF - Date fin reelle])

I can recover the previous date but not always.
How can I fix this problem

Thanks