Thanks RolandP for your precision, the first day of validity can be on any date.
However, the Swiss Half-Fare Card is valid for 1 month, as indicated in T673:
4.2.1.1 Swiss Half-Fare Card, valid for 1 month.
In my opinion, it means that the number of days depends of the first day of validity and is not fix, for example only 28 days between 15.02.2025 - 14.03.2025.