Problems with opening and closing balance

So, my issue is rather simple. I have a list of products, let's say

• Product 1
• Product 2
• Product 3

And I have a list of transactions from 2015 up to 2017, around this products, so it looks like this

Date             |  Product   | Quantity | Comment

------------------------------------------------------

2015-01-01  | Product 1 |      5        | Sales  

2016-02-01  | Product 1 |      -2      | Acuisition    

....

...

What I want in the end is a pivot table, containing my products, with 3 columns: 'opening balance', 'period's change', 'ending balance', with a slicer on a date (month, year, day, anything), which will allow me to see the balance of each product for every day, every month of every year.

What I have tried:

• Using TOTALYTD , it works perfect, but just gives me balance for only the selected year. I need all of them
• Combining last method, also creating a parameter in power query, to pre-agregate previous year data, and store them as "previous year closing balance", yet parameters are not convinient to control
• Using cumulative sum with something like this

Starting Ballance:=CALCULATE(SUM([Qty]),all(Transactions[Date]),DATESBETWEEN('Calendar'[Date], min('Calendar'[Date]), min(Transactions[Date])-1))

Basically I sum up all the transactions that happened before the filtered date, simple. This works perfect except the cases, where there were no transactions with a particular item in the selected period, so I get a blank value inside the formula and an error occours. 

Also tried this solution, had the very same result.

I know that there is a nice thing called "time intelligence" in power bi system, but all it has for the moment limits me inside one year, there are no "lifetime" functions. 

Looks like I need something like "last non blank" but I can't figure out how to implement it here. 

It'd be veeeery nice if anyone could help me in this, I have a feeling that I'm just missing something rater simple.