Calculate a dynamic value each test

Hi there!

 

I would like to use a variable which should be calculated each test. 

For example, I would like to use a particular date (e.g. 01.05.2019).

I choose the "Keys" operation and set 01.05.2019 as Value. 

Next time I want to use another value (e.g. 01.06.2019) but it should be updated automatically. 

 

How can I set a rule to update my current value automatically and add one month each test like:

 

Thank you in advance!