Your question is raised by the Turei Even in Rosh Hashanah. He explains, based on the Gemara in Sanhedrin, that there are two possibilities of redemption derived from the verse, "Ani Hash-m b'Itah Achishenah" -- "I, Hash-m, will precipitate it at its time" (Yeshayah 60:22). Thus, there is a specific time when Mashiach is supposed to come, and the Tana'im argue about when this is whether it is in Nisan since that is the Leil Shimurim, the special time for generations when Jews are to be redeemed, or in Tishrei when the world is judged and given a new task every year. However, at all times Jews must believe that there can be "Achishenah" and Hash-m will precipitate the redemption before its destined time and this could happen at any time during the year.

The Mishmar ha'Leviyim suggests that since we do not know how Mashiach will develop until it actually happens (Rambam, Hilchos Melachim 12), it isn't our business to explain all the statements about the date when he will come but rather to believe that he can come at any time.

Yoel Domb