We mentioned above the Mishnah in Succah which teaches that R’ Yochanan ben Zakai made a takonah that chodosh is forbidden the entire day of the waving of the korban omer. We also explained that this is because: מהרה יבנה בהמ ״ ק – we must anticipate the speedy rebuilding of the Beis HaMikdosh, and we don’t want people to make mistakes.
R’ Moshe Feinstein (Igros Moshe, Orach Chaim 5:8) proves from this sugya that the obligation to be mezapeh liyeshuah, is not something which is a doubt, rather, it’s an obligation to anticipate the arrival of Moshaich with certainty. If it was only a doubt, and only a small doubt, what strength would R’ Yochanan ben Zakai have to forbid people from eating chodosh an entire day, when on a de’O’raisa level during a time when there is no Beis HaMikdosh and no korbonos, chodosh is permissible already from sunrise?
From the fact that R’ Yochanan ben Zakai had the strength to do this, it must be that the obligation to anticipate the arrival of Moshiach is something we have to do with certainty (צ״ ל דיש חיוב כל יום ויום לצפות כעין ודאי שיבא היום). Once anyway there is an obligation to eagerly await Moshiach’s arrival every single second with certainty, it’s understandable that R’ Yochanan ben Zakai has the strength to make such a takonah. (See also Dibros Moshe, Gittin 18a, he’orah 25)
