The mishna discusses various cases of me'chusar z'man and cites a machlokes between the Rabbonon and Rav Shimon whether there is a violation of schechting outside the beis hamikdash when at the moment it is unfit to be brought as a korban inside. The implication is that if one were to be makdish an animal that is me'chusar z'man (either before the 8th day or the day that the mother was schechter) or a bird that is me'chusar z'man (Torim before they are old enough), the hekdesh status would be binding. Tosafos (d.h. heter) questions this because the gemara 12a says that only the night before the 8th day can one be makdish it, which implies that before that time the kedusha status wouldn't take effect. Tosafos concludes that although there is a prohibition to be makdish an animal that is me'chusar z'man, the kedusha status would indeed take effect. Tosafos writes that even according to Rava in Temura 4b who says that when the Torah says not to do something it is generally not binding, this would be an exception to the rule - the makdish would be in violation even though the hekdesh will take effect.
The Minchas Chinuch (293) says that the Rambam seems to concur with Tosafos on this point. The Rambam (Ma'aseh Korbanos 18:10) compares being makdish an animal before the 8th day to being makdish a ba'al mum. Therefore, just as by being makdish a ba'al mum the status is binding, so too being makdish a mechusar z'man the status is binding (others assume that according to the rambam the hekdesh isn't binding). However, Rashi in Bechoros 21b (d.h. lei'lif) understands that the hekdesh isn't binding at all. The Shita Mikubetzes (zevachim 12a) also writes that before the night of day 8, the kedusha will not be binding. He questions how kedusha can be binding an a fetus, it should be no better than me'chusar z'man? The Shita Mikubetzes answers that kedusha of the fetus is binding together with the mother, or that the p'sul of mechusar z'man only begins at a time when it is fit to be sacrificed - at birth.