SK
“Today’s generation and for the last thousands of years we change the year number to a new year in Tishrei with the new Year starting and counting Rosh Hashanah as the start of the new year ”
Incorrect you are mixing up two things
The year starts w/ Tishrei (for most things) Yet Tishrei is still the Seventh month.
Tishrei is called the Seventh month even though it is the first month. In the torah the months don;t have names. Much like days of the week We use sunday Monday etc. In Hebrew today the months don;t have names just yom rishon/Day one yom shieni Day two etc Except for shabbos.
with me?
so Biblical months
wha twe call tishrei is chodehs hahshivii the Seventh month. This was and remains the name of the month. Even if the New year begins in tishrei, it still begins with chodesh Hasivii the seventh month
(Lehavdil September is the ninth month even though it is called seven/sept – lehavdil not a perfect comparison don;t get caught up on it.)
bottom line is Av is never called 11, it MIGHT even be a bitul aseh to do so.