Hello Yienoixee64
Thank you for your enquiry
The Swiss Travel Pass Flex is valid for a specific number of days (3, 4, 6, 8, or 15) within one month from the first day of use, not necessarily tied to a calendar month. Therefore, you travel days can indeed be spread over two calendar months as long as they fall within the one-month validity period from the first day of use.
Have a good day
Kind regards
GiuseppeL