In my sefer nasiach b'chukecha (page 3), I mentioned that Tosafos in Baba Basra implies that zerizin applies even before the z'man of the mitzvah comes, which seems to contradict other places where the concept of zerizin seems to only apply once the obligation of the mitzvah exists. See also page 103 for a discussion of when zerizin applies.
I see now that i was mechaven to R' Elchonon (kovetz shiurim 28) who asks this question. How can Tosafos suggest that one would pay early by pidyon ha'ben to avoid a tirda even though no specific person can claim the money from him due to zerizin makdimin, there is no obligation at all before 30 days, and it is not even possible to fulfill the mitzvah within 30 days? If for example the money would not be in the hands of the kohen at the end of the 30 days, the pidyon won't take effect, so why should we assume that a person would pay early? Why should zerizin apply prior to the time of the mitzvah?
Perhaps Tosafos holds that although zerizin wouldn't make sense by pidyon haben within 30 days, it would apply immediately at the conclusion of 30 days. Meaning, the din zerizin doesn't require paying 5 selah within 30 days, but it would require making sure that as soon as the z'man mitzvah comes, it has already been taken care of. By most mitzvos where the ma'aseh hamitzvah is THE mitzvah, it is impossible to set it up early so the zerizin wouldn't apply at all until the z'man mitzvah arrives, but by pidyon we find that if it was paid within 30 days and is still around at the end of the 30 days, the pidyon takes effect. Therefore, Tosafos may hold that the din zerizin would require of him paying within 30 days so that as soon as the z'man comes the child is automatically redeemed at the earliest possible moment. This concern would create a tirda within 30 days to pay. However, the minhag is clearly not like tosafos because we don't find practically that anyone pays the pidyon haben within the 30 days. The proof against tosafos is the gemara in pesachim 4a by avrohom where he got up early in the morning but didn't travel the night before. Although he could have made the preparations by travelling the night before to enable himself to do the mitzvah as early as possible, Avrohom held that the din zerizin didn't require him to do that (see what i wrote in nasiach b'chukecha page 103 regarding the issue of zerizin the night before).