Last year, I was in the Me'ah Sha'arim neighborhood of Jerusalem during much of the Fall haggim. The Seventh Month (Tishrei) coincided with the Muslim month of Ramadan, during which Muslims fast during daylight hours. A cannon fired in the evening indicates the time when it is permissible for them to eat.
That year, I clearly recall hearing a second cannon fired a good 45 minutes to an hour after the first. I asked my friend if he knew what the second cannon was for.
But, then I realized was it for on my own. The second fired much later after sunset were for the Muslims who hold by Rabbenu Tam.
Rabbenu Tam is well-known for many of his rulings on Jewish Law, including his stringent ruling regarding when Shabbath ends.