Storing calculated value as constant

Hi Greg!

I'll try to explain on simple example. Suppose we have this source Data:

Date              Item          Quantity

1/1/2018       Apple              1

1/1/2018       Apple              2

1/1/2018       Apple              1

2/1/2018       Banana           5

2/1/2018       Apple              3

2/1/2018       Banana           1

2/1/2018       Apple              4

3/1/2018       Banana           3

There are three dates: 1/1/2018, 2/1/2018, and 3/1/2018. Suppose I want to compare iteam for the first two dates. 

I create visual: filter with dates only 1/1/2018 and 2/1/2018. And I want to understand which dates out of three were selected. I create two measures:

Maximum = MAX(Data[Date]) and Minimum = MIN(Data[Date])

So, because I have filter set up, these two measures if used should show Maximum = 2/1/2018 and Minimum = 1/1/2018.

Next I want to calculate the quantity of items for each date:

Maxmonth = CALCULATE(IF(Data[Date]=[Maximum],SUM(Data[Quantity]),BLANK())) - do not have access to PBI now, the logic should be something similar to this

Minmonth = CALCULATE(IF(Data[Date]=[Minimum],SUM(Data[Quantity]),BLANK()))

ALso, I want to see difference: Difference = [Maxmonth] - [Minmonth]

Now I want to create the comparison table visual (pivot) with the following columns:

Item   |    First Date   |    Second Date   |   Difference

If I add everything to table, here is what I get for Apples:

Item   |    Minmonth (1/1/2018)   |    Maxmonth (2/1/2018)   |   Difference

Apple                     4                                       7                           -3

For apples everything is as expected, no problems. However, here is what will be with bananas:

Item   |    Minmonth (1/1/2018)   |    Maxmonth (2/1/2018)   |   Difference

Banana                 6                                       6                            0

But expected values are: 

Item   |    Minmonth (1/1/2018)   |    Maxmonth (2/1/2018)   |   Difference

Banana                                                         6                            6

The problem is, when it comes to bananas, it filters out everything for bananas, and in the Date column we have only one date left, which is 2/1/2018. Therefore, it is both Maximum and Minimum value for bananas. - this is the problem

I do not want it to work like that. I want to calculate Maximum and Minimum, and somehow make them constant, so that they are not affected by items filtering. Or probably there is another simple way to solve this problem?

(I know that I can create in the source table another column withe the first occurence date of the item in the table. However, the problem is that the formula takes looooots of time to be calculated, because I have about 300k+ rows. I need something simpler, with less calculations)

Hope this explanation helps to understand what I need. Thanks!