This Halacha is an excerpt from our Sefer
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Counting on condition:
If for whatever reason one does not desire to count Sefira at a certain time, but is afraid that if he does not count at this time he may forget to count later on, then he may count now without a blessing and have in mind the following condition “If I forget to count at night I am relying on this count to fulfill my obligation. If however I will not forget, then I am now having in mind to not fulfill my obligation with this count.” It suffices to think of this condition in one’s mind, and it is not required to be verbalized.
If one counted without this condition: If one did not explicitly have in mind to make this Tnaiy, to not fulfill his obligation with this count if he remembers at night, and rather he casually counted without any specific intent, then although he did not say a blessing at this time and did not answer Amen after the Chazzan’s blessing, nevertheless he may not count at night with a blessing, as perhaps he has already fulfilled his obligation.
If one will not be Davening Maariv until late and is afraid that he may forget to count Sefira after Maariv, may he recite the Sefira beforehand, using the above Tnaiy?
Yes. He is to recite the Sefira without a blessing using the above mentioned Tnaiy and then repeat it with a blessing upon remembering later at night.
 Admur 489:12 [see also Admur 46:9 in parentheses regarding Tnaiy by Shema]; Michaber 489:3 as explained in Taz 489:6; M”A 489:7 according to opinion that Mitzvos need Kavana; M”B 489:16; See Birchas Habayis 46:15 footnote 41; Piskeiy Teshuvos 489:13 footnote 62; Piskeiy Teshuvos 296:17
Other opinions: Some Poskim rule that making a Tnaiy is useless, as one cannot questionably join the fulfillment of a Mitzvah. [Taz 489:6] It is thus questionable whether he has fulfilled his obligation even if he made a Tnaiy and remembered later on. [P”M 489 M”Z 6]
 Admur ibid; M”A ibid
The reason this Tnaiy helps even according to those who rule the Mitzvos do not need intent: Even according to the opinion who holds that that Mitzvos do not need intent to fulfill ones obligation, if one specifically has in mind to not fulfill his obligation through this counting then everyone agrees he is not Yotzei against his will. Therefore, if one remembers at night he is to count with a blessing. [489:12; Rama 489:3; M”A 489:8; Chok Yaakov 489:14]
 Admur ibid; M”A 489:8
 The reason: As he fulfills his obligation according to those opinions who say that Mitzvos do not need intention for one to fulfill his obligation. Furthermore, even according to those Poskim who argue that Biblical Mitzvos require intention, today the Mitzvah of Sefira is Rabbinical, and there are Poskim who rule that by a Rabbinical Mitzvah according to all one does not need intention. [Admur ibid]