If you haven't counted sefirat haomer yet that night, you're not supposed to say the current day (for example, 'today is the 5th day') before making the bracha. But what about saying what today is in an indirect manner, such as, 'today is the square-root of 25th day', or 'today is the 10 minus 5th day'?
That is why we say "yesterday was the fourth day" before counting. You do not want to say "today is" in any way because even in an indirect manner you are counting today. Once you have said "today is" then however you say the number, that is still a count.
"Code of Jewish Law Ganzfried - Goldin, volume 3 page 52 chapter 120 number 3 (translation of Kitzur Shulchan Aruch). Any mention of the current number forbids the bracha. This is lechatchila. There is an answer that you can say it b'diavad.
Advance notice of the day of the Omer? quotes the Shulchan Aruch Orach Chaim 489:4
There are those that say that you can express the count using a math problem and still say it with a bracha. The reason appears to be that you are in effect saying that you do not want to be yotzi with what you are saying. See Counting Sefiras ha-Omer Unintentionally Note that the implication of what is written below seems to be that if you say the bracha and then count in one of the ways written below, then it is considered a valid count.
Question: If, inadvertently, one forgot and responded with the correct sefirah count, is there any way that he can count again that night with the blessing?
Discussion: B’diavad, one is permitted to recite sefirah that night with the blessing:
- If he responded by saying just the correct number of that day, but
did not say “Today is number so-and-so,” then he may repeat the
sefirah with a blessing. But if he omitted just the word “ba-Omer” (or “la-Omer”), then the count remains valid and it may not be
repeated with the blessing.
- If he responded by saying, “Today is so-and-so” but did not mention the “weeks” count, he may still repeat the sefirah with a blessing. For instance, on the seventeenth day he responded, “Today is day number seventeen,” but he did not add, “which is two weeks and three days.” [Obviously, this applies only after the first week of sefirah has passed.]
- Even if he responded with the correct number and the right weekly count but had specific and clear intention not to fulfill the mitzvah of Sefiras ha-omer with his response, then he may repeat the sefirah with a blessing.
- If the person who inadvertently forgot and responded, “Today is so and so” is one who is always particular to count the Omer after tzeis ha-kochavim only, and this exchange took place before tzeis ha-kochavim, he may repeat the count with the blessing.
- If on the fifth day, for example, he responded, “Today is six minus one,” or, “Today is three plus three,” he may repeat the count with the blessing Be’er Moshe 3:82.
- If in response to the question he wrote down the correct sefirah count (but did not say it), he may repeat the sefirah with the blessing.
- If the questioner, for example, asked, “Is today day number five?” and the response was, “Yes, it is,” then both the questioner and the respondent can repeat the sefirah and recite the blessing.