Set FILTER value with variable

Hello.

I'm ex QlikView developer switching to Power BI. Now I'm trying to do some aggregations I've used to do in QlikView. I'm missing badly especially this one - to set filter value in expression using variable. Here is an example:
CALCULATE(
SUM('Customer Transactions'[Amount]);
FILTER('Customer Transactions'; 'Customer Transactions'[Transaction Date]<"Some specific date")
)
In QlikView I was able to define this "specific date" in two ways:

1.  Based on selection in [Transaction Date] field. In this case "Some specific date" = MAX(Transaction Date). To make this working you should remove [Transaction Date] selection effect in the expression.
2.  Define variable varDate with empty value. User is able to set value to varDate via calendar object. In this case "Some specific date" is just varDate value.

I've found some solution about point 1 here http://goo.gl/XvvMs9 but it's far too complex. I'm trying to find a way to make point 2 in Power BI. Is there such possibility?