Yesterday I mentioned the view of Tosfos (Menachos 66a) that since sefrah is derabbanan it can be counted during bein hashemashos based on the principle of safeik derabbanan l’kula. The Ran disagrees and argues that sfeika derabbanan l’kula means that if one is forced into a situation of doubt, one can be lenient; it does not give a person the right l’chatchila create a situation of safeik.
Rav Hershel Shachter (article here) notes that the Ran’s point seems to be borne out by the principle of ain m’vatlin issur l’chatchila, which tells us that one should ideally avoid having to decide a safeik if such a situation could be avoided. Why then does Tosfos advocate counting during beis hashemashos?
Rav Shachter suggests that Tosfos agrees in principle to the Ran, but argues that the case of sefirah is different because there is compelling reason in this case to enter into the safeik: the din of temimos. Tosfos held that the count of sefirah must be done at the earliest possible moment to include as much of the 24 hour day as possible. (If one davens ma'ariv at a late minyan it would be preferable based on this to count sefirah earlier.)
Why does the Ran disagree? The Ran may simply hold that since were sefirah d’oraysa counting early would be impossible (as sfeika d’oraysa l’chumra), the din derabbanan of sefirah done zecher l’mikdash cannot have more stringent parameters the d'oraysa mitzvah and cannot require counting earlier. Altrenatively, Rav Shachter suggests an interesting chiddush: the Ran may hold that the count of sefirah done at any point during the night covers the entire time period in question. In other words, just like kiddush hachodesh by beis din, even if done mid-day, retroactively creates a status of kedusha on the entire day, so too, the count of sefirah done at any point during the night effectively includes every moment of the night in the mitzvah, retroactive to the night's beginning. According to this approach, the reason to count early is not because of the din of temimos, but rather because of the usual din of zerizim makdimim.
A similar machlokes perhaps exists with respect to the mitzvah of kiddush. The Tur writes that one should hurry to recite kiddush immediately upon getting home Friday night to fulfill the mitzvah of zachrey’hu b’knisaso. The Rambam (shabbos 29:14), however, makes no mention of a need to rush, and even adds that although “ikkar kiddush” is at night, the kiddush may be recited at any point during Shabbos.
According to the Tur, the mitzvah of kiddush serves to sanctify the time of Shabbos going forward from the moment it was recited. Therefore, to encompass as much of Shabbos as possible, kiddush must be recited at the earliest possible moment. The Rambam, however, understood that kiddush works even retroactively, like the kiddush hachodesh of beis din. There is no specific din in hilchos kiddush that demands that it be recited early, only the usual din of zerizim.