I'm borrowing heavily from the other answers, but I'm putting everything into one place.
A: The order of Romeos passenger in the Greek alphabet.
The answer is
1,
because
Alfa Romeo is a car manufacturer; cars have passengers, and alpha is the 1st letter of the Greek alphabet, associated with Romeo.
Thank you to @rand-althor.
Second clue
B: The two last digits in the year the returning Lex Luthor was awarded his (number of clue A) Oscar.
The answer is
96,
because
Kevin Spacey played Lex Luthor in Superman Returns and was awarded his 1st Oscar for his appearance in The Usual Suspects in 1995. He was awarded this Oscar in 1996.
Thank you to @rand-althor and @dmg.
Third clue
C. The man in B didn't manage to complete here on time, and could have called in his complaint on this number, in Apple time!
The answer is
311
We've identified The man in B as Kevin Spacey, who attended the Juilliard School in New York City to become a comedian, but left before receiving a diploma.
If you have a complaint in NYC, you can dial 3-1-1.
Thank you, @CDSpace.
Fourth clue
D. The street in the city of C where marble is buried.
We've identified the city of C to be New York City, so the answer is
2
because of the marble cemetery located on
2nd Street.
Thank you, @rand-althor.
Fifth clue
E. A European convention was held in a city in Italy sometime in the 20th century, and they discussed a partial golden retreat. In March 2007 the football club from the same city was in the (number of clue D) league. The two last digits of the year before, and the two last digits of the year after the conference is the number of this clue.
The answer is:
21 and 23
because
The Genoa Conference was held (in Genoa) between representatives of 34 countries in 1922 to discuss the global economic situation following World War I. Genoa CFC played in the 2006/2007 Italian Cup, Serie B.
Sixth clue
F. In the episode number (E) of Sesame Street, Kermit the frog appears this number of times.
The answer is
0
because
Kermit made no appearances in episode 2123 of Sesame Street.
Put these clues together and you have a date and time.
1 + 96 + 311 + 2 + 21 + 23 + 0 → 1963-11-22 12:30
It was at 12:30 CST, November 22, 1963, when President John F. Kennedy was shot twice by Lee Harvey Oswald.