Responsa do Powtórzonego Prawa 16:9

Answer: The early rabbis (Rishonim), may their memory be blessed, already spoke about this. And the Ran (Rabbenu Nissim) wrote in the Laws of Sukkot - and these are his [words] - "I have seen someone write that if one does not count at night, he may not count during the day; as it is taught (in a mishnah), 'The entire night is suitable for the counting of the omer.' It is implied that during the day, however, it is not. And because of that, we do not recite the blessing over time (shehechaynu)." To here [are his words]. And it appears to me that the reason for the thing is that it would be fine if when he did not count at night, he could count during the day - it would come out that its time is set and all of the fifty days would be one time. So he would be able to recite the blessing, shehechaynu. But now that if he did not count at night, he could not count during the day - it comes out that this day has no possibility of being made up. So how could he recite the blessing [which includes the words], "we have reached this time," for the counting of fifty days? For he has not counted all fifty! And that which he goes back to counting on the next night - that is a different time. And it was impossible to decree that he should recite the blessing, shehechaynu, at the beginning of the counting - and if he forgets and does not count a night, he goes back and recites the blessing, shehechaynu, again on the next night; and likewise each time he forgets a day. As behold, it is all one commandment; and one may not recite this blessing twice upon [the same] one commandment. Regardless, concerning the essence of the question, I say [about] any commandment which is a preparation for another commandment - they decreed that one should recite the blessing, shehechaynu, on the main commandment, and take care of both of them. Hence, the counting of the omer is a preparation for [Shavuot], as it is written (Deuteronomy 16:9-10), "Seven weeks count for yourself; from the time the sickle begins felling the standing grain, etc. Then you shall observe the Feast of Shavuot for the Lord, your God." So it counts for this and for that. And like this do we say about the making of a sukkah, such that there would be a need to recite the blessing, shehechaynu, at the time of its making. And even so, we only recite a blessing upon it at the time of the commandment, which is the time of the festival, and it counts for this and for that. Moreover, it is because the end of the commandment is only at the end of the counting. And hence it is impossible to recite the blessing, shehechaynu, at the beginning of the counting, as it is only a part of the commandment; nor at its end, as we do not recite the blessing, shehechaynu, at the end of a commandment. And the Rashba wrote that we do not recite the blessing, shehechaynu, on something that is [a cause for] anguish. And I have written what appears correct according to the poverty of my knowledge. David ibn avi Zimra.