Making calculations is perfectly acceptable according to many great rabbis of the past, and many did exactly that. For example, according to the Abarbanel, it is only forbidden to make the calculation based upon astrology; however, it is permissible to calculate a date based upon Tanach (Ma’ayeni HaYeshuah 1:2). The Ramban held that the prohibition of the Talmud only applied to earlier generations; now that we are on the eve of redemption, there is no prohibition (Sefer HaGeulah, Ma’amar 4).
The Malbim concurs, and provides the following analogy to explain his opinion: The situation is like that of a father and son traveling a long distance. As they start out, the son begins to ask when they will arrive, and of course the father does not answer. However, as they near the town, the son asks the same question, and this time the father readily answers that it is only a short while before they reach their destination. So too it is with us: now that the time is clearly approaching, we cannot help but notice and interpret the signs all around us that tell of the impending geulah ... As the time of the keitz grows nearer, the doubts will become smaller, and at the keitz, all doubts will be removed ... As the time grows closer, the uncertainty recedes in the wake of the increasingly “abounding wisdom” (Introduction to Daniel). The Maggid of Dubno used a similar analogy as well.
The Zohar even states that it is not God’s will to reveal the arrival date of the Moshiach, but when the date draws near, even children will be able to make the calculation (Bereishis 118a). According to the Vilna Gaon, there seems to be little problem making the calculation from his commentary, but one who does must promise not to reveal his finding to another: “And from here [what I have just written] you can calculate the time of the Final Redemption if, God forbid, we do not merit [to bring it earlier]; however, I have imposed an oath, in the name of the God of Israel, on the reader of this that he should not reveal it.” (Biur HaGra, Sifra D’Tzniuta, Chapter Five)
