Measure to Calculate Difference vs. Previous Period

This isn't the prettiest DAX, but it should get the job done for you:

LastExtr =
Var SecondToLast = MAXX(Table2, MAXX(FILTER(ALL(Table2[Extract Date and Time]), Table2[Extract Date and Time] < EARLIER(Table2[Extract Date and Time])), Table2[Extract Date and Time]))
return CALCULATE(SUM(Table2[Value]), ALL(Table2[Extract Date and Time]), Table2[Extract Date and Time] = SecondToLast)

diff01 =
VAR Last = CALCULATE( SUM( Table2[Value] ), FILTER( Table2, Table2[Extract Date and Time] = MAX( Table2[Extract Date and Time]) ) )

return
if([LastExtr] & "" = BLANK(), BLANK(), Last - [LastExtr])

Then diff01 should give you what I think you wanted, the difference between a given extract and the extract immediately before it.

The last if statement in diff01 just stops you from getting weird/garbage values for the oldest period in the document. If you'd like, you can replace it with just Last - [LastExtr]

How's this as an updated version of LastExtr (the other one doesn't have to change):

LastExtr =
Var SecondToLastOrFirst = IF(HASONEVALUE(Table2[Extract Date and Time]), CALCULATE(MAX(Table2[Extract Date and Time]), FILTER(ALL(Table2[Extract Date and Time]), Table2[Extract Date and Time] < MAX(Table2[Extract Date and Time]))), MIN(Table2[Extract Date and Time])
)
return CALCULATE(SUM(Table2[Value]), ALL(Table2[Extract Date and Time]), Table2[Extract Date and Time] = SecondToLastOrFirst)

Here's the result it gives now:

Okay, phew took a little debugging but you solution(s) work great. I think next time I'll stick with time intelligence functions and forget about the users who want to custom select [Export Date and Time], but this was a really usefull exercise. 

As a note, for those looking for similar funcaitonality and/or expansion of the "Quick Calc" functionality the idea is suggested here:

https://ideas.powerbi.com/forums/265200-power-bi-ideas/suggestions/14476374-percent-change-percent-difference-in-quick-calc

EDIT: jahida you may want to edit your last post (solution) to include the [diff01] measure as that was in a previous potential solution. Thanks Again.

v-qiuyu-msft See the attached, what if my goal was to see the difference between each of the period (from the period previous to it). I believe some of DAX time intelligence functions would be helpful here; however, because the Extract Date & Time is not done at a consistent interval I'm not sure they can be leveraged....

difference-two dates_MDGedits02.pbix

As mentioned, Tableau makes this delta/difference calculation very easy where as PBI's only "Quick Clac" functionality is to show values as a % of the grand total. In the below image on the first row shows a difference of $2.097M for the period between 7/27/16 10:40:00AM and 7/28/2019 8:54:00AM and a difference of $0 for the subsequent period. 

jahida working through this now... out of curiousity for you SecondtoLast Variable... is there any benefit to using MAXX like you did....

 SecondToLast = MAXX(Table2, MAXX(FILTER(ALL(Table2[Extract Date and Time]), Table2[Extract Date and Time] < EARLIER(Table2[Extract Date and Time])), Table2[Extract Date and Time]))

vs.

SecondToLast= CALCULATE(MAX(REF_Opportunites[Export_Date_&_Time]), filter(REF_Opportunites, REF_Opportunites[Export_Date_&_Time]<>MAX(REF_Opportunites[Export_Date_&_Time])) )

?

jahida working through this now... out of curiousity for your SecondtoLast Variable... is there any benefit to using MAXX like you did....

 SecondToLast = MAXX(Table2, MAXX(FILTER(ALL(Table2[Extract Date and Time]), 
Table2[Extract Date and Time] < EARLIER(Table2[Extract Date and Time])), 
Table2[Extract Date and Time]))

vs.

SecondToLast= CALCULATE(MAX(REF_Opportunites[Export_Date_&_Time]), 
filter(REF_Opportunites, 
REF_Opportunites[Export_Date_&_Time]<>MAX(REF_Opportunites[Export_Date_&_Time])) )

?

Probably not, just that I'm not great at using the Calculate function. Your version looks fine, and probably more efficient (except maybe needing an ALL on the Extract Date and Time column, not sure about that. If you did use an all, you'd need to change the <> to <).

jahida everything works pretty well with your solution, thank you.... except the total's value going across the rows doesn't really make sense; for Cat1 for the period in your image the difference should be 401 not -100. Any soltion here or is this a limitation of the Matrix Visual?

Edit: Yes, it would have value, and yes 401 not -401.

Yeah the total basically just gives the same value as the latest date. You're right, best to hide it. You'd have to write a separate case in the measure (if there's more than one date in the current context, probably using HASONEVALUE) in order to get a result like 401 (I feel like it should be positive but could be wrong). I can experiment if that's useful to you, but I'm glad you found the rest of it useful.

EDIT: Your edit makes it seem like this is a decent priority to you, so I'll give it a shot.

jahida I just walked away from my laptop but looking at your SecondToLastOrFirst variable... are you sure that would work if you have more than two periods of data selected? (eg. 3 extract dates?)

Pretty sure, probably worth some additional testing (a bigger dataset than the one you provided) but I think it would be fine.

I threw in a tad more data and it works fine:

jahida I'm getting some odd values.... if I store the SecondToLastOrFirst Variable as a Measure then it returns the MIN() of my Export Date & Times.

Does you solution continue to work if you add more data prior to the dates/periods you have selected (and then don't select those dates)? If so, let me know and I'll produce a .pbix we can both work from duplicating my issues. 

Not sure what you mean so maybe a .pbix would be best.

This is too many rows to paste into a Pbix data table, for similicities sake can you see if your method will work with the linked excel document as the data source?

ChangefromPervious_sampledata.xlsx

You result should look like this for Cat1, Cat2, and Cat3 respectively with the following 5 most recent periods selected:

Or in Total.... Cat1 = ($530,242) Cat2= $769,142 and Cat3 = $300,000