This Halacha was recorded in 2010. The numbers below are for 2010, but can be changed for the current year.
There is a custom mentioned by several Halachic authorities (including the Kenesset Ha’gedola, the Hemdat Yamim and the Zibheh Sedek) to dim the lights in the synagogue on the night of Tisha B’Ab after the reading of Echa and the Kinot, and to announce how many years have passed since the destruction of the Second Temple. The Hazan makes this announcement in a somber, subdued voice. This custom has been practiced for centuries in Sephardic communities around the world, and it is our practice, as well. In some communities, all the lights of the synagogue are turned off, and only the Hazan has a small light so he could read the text.
There are two formulas that one can use to easily calculate the number of years that have passed since the Hurban (destruction of the Temple). The first is the phrase “Hayim Mi’Yeshu.” This means we take 68 – the numerical value of the word “Hayim” – from “Yeshu,” the Gregorian calendar, which begins with the birth of Yeshu (founder of Christianity). Thus, for example, in the year 2010, we would subtract 68 from 2010, and arrive at 1942, which is the number of years from the Hurban until 2010.
Another formula is to add 172 – the numerical value of “Ekeb” – to the year of the Jewish calendar. In the year 5770 (2010), we would calculate 770 + 172, which equals 942. We can then just add 1,000 to arrive at the number.
In any event, announcing the number of years is a time-honored tradition which should be followed.