10 Tishrei 5711 = 21 September 1950
Yom Kippur is not a commemoration of an event. It is a day of fasting, reflection, and prayer for forgiveness for the sins of the past year. The Torah set the date for Yom Kippur on the tenth of Tishrei in the Jewish calendar. According to the Gregorian calendar, Yom Kippur falls in September or October.
The Bengali date 2 Ashar 1370 corresponds to July 16, 1963, in the Gregorian calendar. Bengali dates are based on the Bengali calendar, which is roughly 594 years ahead of the Gregorian calendar.
The Bengali date 21 Boishakh 1387 corresponds to May 4, 1980, in the Gregorian calendar. The Bengali calendar is approximately 594 years ahead of the Gregorian calendar, which is why the conversion yields this date.
The date 23 Katak 1985 corresponds to December 8, 1985, in the Gregorian calendar. Katak is a month in the Nepali calendar, which typically falls around November to December in the Gregorian calendar.
In the Gregorian calendar, December 31st
31 May 1949.
The Bengali date 18 Falgun in the year 1395 corresponds to March 2, 1989, in the Gregorian calendar. The Bengali calendar is approximately 593 years ahead of the Gregorian calendar, which is why the conversion results in this specific date.
19 September 1979
The Julian calendar was used in most places that use the Gregorian calendar today, especially in countries considered "Christian". The switch to the Gregorian calendar, which is very similar but 25 times more accurate, began on October 15, 1582 (Gregorian date).
Some places were still switching from the Julian Calendar during the 20th century, but the first group of countries to switch to the Gregorian Calendar did so on October 15, 1582.
The Bengali date 16th Falgun 1398 corresponds to March 1, 1992, in the Gregorian calendar. The Bengali calendar is approximately 593 years ahead of the Gregorian calendar, and Falgun typically falls around February to March.
The date not only changes each year it changes daily. That is part of the purpose of a calendar.