The way I read it and also the way it seems to work is that you can only have one of those 2 bonus calendars AT SAME TIME.
This brings some questions since the game information isn't great.
As you can only see the info about 1 bonus Calendar at same time if you scroll down, many people only realise it when they go for the 2nd free calendar and saw that message about discarding the previous one. It is working as intended (the previous should be forever dismissed)?
Its not clear if it is possible to run one of the calendars for X time and then switch to the other. This question is even more relevant if we verify that their duration/validity is greater than 31 days (more specifically, 45), which makes it possible to collect 15 days from one calendar and then 30 days from the other. Can we do this?
If the intended idea was to force people to choose between the 2 bonus calendars and discard the other, then that should be done in a total different way (one pack on the store with a selection inside it). Anyway, some clarification need to be done or people may found themselves in a situation where the final calendar wasn't the one they want.
Can anyone clarify all this cause the current status can lead people to several kind of mistakes.
Thanks