Context: During the period between 1582, when the first countries adopted the Gregorian calendar, and 1923, when the last European country adopted it, it was often necessary to indicate the date of some event in both the Julian calendar and in the Gregorian calendar, for example, "10/21 February 1750/51", where the dual year accounts for some countries already beginning their numbered year on 1 January while others were still using some other date. Even before 1582, the year sometimes had to be double dated because of the different beginnings of the year in various countries. Woolley, writing in his biography of John Dee (1527–1608/9), notes that immediately after 1582 English letter writers "customarily" used "two dates" on their letters, one OS and one NS.

Question: When did the last country to adopt the Gregorian calendar start using it?
Answer: 1923

Question: At what date did the first country start using the new calendar?
Answer: 1582

Question: How which calendar did some countries notate the date?
Answer: in both

Question: Who said that English writers usually used both dates on writings?
Answer: Woolley

Question: What difference in dating caused the use of double-dating documents?
Answer: different beginnings of the year

Question: What happened in the fifteenth century that made it necessary to give dates using both calendars?
Answer: Unanswerable

Question: What happened in the nineteenth century the Gregorian calendar that made it necessary to give dates using both calendars?
Answer: Unanswerable

Question: Why was it not necessary to use the dual year when giving two dates?
Answer: Unanswerable

Question: Why did the year sometimes have to be double dated after 1582?
Answer: Unanswerable

Question: What did John Dee note?
Answer: Unanswerable