The prophet Zecharia,  who lived at the beginning of the Second Temple Period, states (Ch. 8) that all of the 4 fasts,  commemorating the Destruction of the Temple shall become days of joyous festivals and gladness.  At what historical time was this prophecy fulfilled?

Rabbenu Chananel states that this prophecy was fulfilled during the Second Temple Period. The Rambam disagrees and states that Tisha B'Av, unlike the other 3 fasts, continued to be a day of fasting and mourning during the Second Temple time.

The First Temple, built by King Shlomo, stood for 410 years. It was destroyed by the Babylonians in 586 B.C.E.  After 70 years of exile in Babylon, the Second Temple was built and stood for 420 years until it was destroyed by the Romans in 70 C.E.

How can there be a disagreement between Rabbenu Chananel and the Rambam over the historical fact of the observance of Tisha B'Av during the Second Temple Period?
Either people fasted or feasted on Tisha B'Av during the Second Temple time. All one has to do is to check the historical and oral religious tradition of that time.

The Talmud in Eruvin 13 states that when great Torah scholars disagree a Heavenly Voice proclaims, "These and those are both the Words of the Living G-d." How can we apply this concept to resolve the difference of opinion between Rabbenu Chananel and the Rambam?

The answer is that Rabbenu Chananel and the Rambam were talking about two different groups of people. The Book of Ezra states that when the Second Temple was built, the old generation, who remembered the First Temple were weeping. The young generation who never saw the First Temple was rejoicing.

The Talmud in Yoma states that the Second Temple was inferior to the First Temple in the following ways – It lacked the revealed Divine Presence, the Holy Ark that contained the Tablets of the Ten Commandments, and there was no king of the Davidic Dynasty. Why were these elements missing from the Second Temple? The answer is found in Ezra Ch. 2. When Ezra called on the Jews to make Aliya from Babylon (Long Island) and Persia only 42,360 responded to his call. The millions of Jews remained behind in exile. There were not enough Kohanim and Levites to serve in the new Temple.  Therefore the Second Temple was pale by comparison with the First.

Thus, the old generation who remembered the splendid glory of the First Temple  were weeping when they saw the inferior quality of the Second Temple. The young generation, who did not have the memory of the glory of the First Temple, rejoiced at the sight of the Second Temple.

Therefore, both the Rambam and Rabbenu Chananel are correct. Rabbenu Chananel is referring to the young generation, who rejoiced and feasted on Tisha B'Av during the Second Temple Period. They grew up in the Babylonian Exile and never experienced the glories of the First Temple. For them, the Second Temple was the zenith of joy, so they educated their children that Tisha B'Av is now a day of joy and celebration.

However, the Rambam is referring to the older generation who continued to fast and mourn during the Second Temple period on Tisha B'Av, because they remembered the splendor of the First Temple, which the Second Temple lacked. They taught their children to continue to mourn and fast on Tisha B'Av.

Thus, both the Rabbenu Chananel and the Rambam are correct because they are referring to different segments of the Jewish People during the Second Temple period.