Question

Have a good week, Rabbi,
I understood that the Rosh’s view regarding the commandment of tzitzit is that a daytime garment is obligated in tzitzit even at night, while a nighttime garment is exempt from tzitzit even during the day. And despite all this, the Rosh still defines tzitzit as a positive commandment dependent on time, and therefore according to his view women are also exempt from it. My question is: how can the Rosh define tzitzit as a positive commandment dependent on time? After all, his entire distinction is between the type of garment that is obligated, and time does not affect the obligation in the commandment.
Best regards,