Flexi-pass validity

Hi,

I bought the flexible-pass 4 days within a months for me and my girlfriend. And I have a question about validity of the pass. Could you clarify if the "month" refers to:

1. A period of approximately 30 days starting from the first day the pass is used (meaning I can use the 4 travel days within the following 30 days after activation).

2. A specific calendar month, where I can only use the 4 travel days within that same month.

For example, if I start using the pass on December 20, does that mean I can only use the remaining travel days within December, or does it count as a 30-day period starting from December 20 (in this example until January 20)?

I’d greatly appreciate it if someone could clarify how this works. Thanks in advance!