To unambiguously specify the date, dual dating or Old Style (O.S.) and New Style (N.S.) are sometimes used with dates. Dual dating uses two consecutive years because of differences in the starting date of the year, or includes both the Julian and Gregorian dates. Old Style and New Style (N.S.) indicate either whether the start of the Julian year has been adjusted to start on 1 January (N.S.) even though documents written at the time use a different start of year (O.S.), or whether a date conforms to the Julian calendar (O.S.) rather than the Gregorian (N.S.). 

The Gregorian calendar was a reform of the Julian calendar instituted in 1582 by Pope Gregory XIII, after whom the calendar was named, by papal bull Inter gravissimas dated 24 February 1582. The motivation for the adjustment was to bring the date for the celebration of Easter to the time of year in which it was celebrated when it was introduced by the early Church. Although a recommendation of the First Council of Nicaea in 325 specified that all Christians should celebrate Easter on the same day, it took almost five centuries before virtually all Christians achieved that objective by adopting the rules of the Church of Alexandria (see Easter for the issues which arose). QUESTIONS:

Q1:
1. What else is dual dating called?
2. What's another expression for dual dating?
3. How else is dual dating referred to?
Q2:
1. What is the function of dual dating?
2. What purpose does the dual dating system serve?
3. What is observed by dual dating?
Q3:
1. How many years are used in dual dating?
2. What is the number of years that dual dating requires?
3. How many years are needed for dual dating?
Q4:
1. Who is the Gregorian calendar named for?
2. What name is important to the Gregorian calendar?
3. What name has been included in the dual dating calendar?
Q5:
1. When was a change instituted establishing the Gregorian calendar?
2. In what year did a change establish the Gregorian calendar?
3. When did the Gregorian calendar come about?
Q6:
1. What was the reason for instituting the Gregorian calendar?
2. Why did the Gregorian calendar come about?
3. What purpose did the change to the Gregorian calendar serve?
