Since FF7's storyline has become more complicated from the presence of extra games and sources, we've found the need to alter the format of the timeline a bit. Now, events take place on a timeline that is either Before Game (B.G.) or After Game (A.G.) which is technically any point in time after the game starts. So when Cloud jumps out of the train in FF7, that's when 0 B.G. ends and 0 A.G. starts. Anything labeled as "SIDE QUEST" is an optional part of the game which may or may not happen, but has no bearing on anything that is not a side quest.

Sources have also been cited in brackets at the end of each entry, so you know where the info comes from and where it was created. Here are their abbreviations.
FF7 Final Fantasy 7 (the actual game)
AC Final Fantasy 7: Advent Children (the CG movie)
DoC Final Fantasy 7: Dirge of Cerberus (PS2 game starring Vincent)
CC Final Fantasy 7: Crisis Core (PSP game)
BC Final Fantasy 7: Before Crisis (Cell Phone game released only in Japan)
OAWTAS:Denzel On a Way To a Smile: Case of Denzel (from the FFVII:AC prologue book)
OAWTAS:Tifa On a Way To a Smile: Case of Tifa (from the FFVII:AC prologue book)

Note that some details taken from OAWTAS:Tifa, OAWTAS:Denzel, and Squall of SeeD's Plot Analysis FAQ have not been double-checked, so if you see something wrong, please let us know.

OAWTAS:Tifa, OAWTAS:Denzel, and Before Crisis were taken from secondary sources that were unofficially translated.

Many Years Ago:

2000 Years B.G.:

Between 1999 - 41 B.G.:

55 B.G.:

c. 40 B.G. :

35 B.G.:

32 B.G.:

30 - 29 B.G.

29 B.G.:

28 B.G.:

25-22 B.G.:

22 B.G.:

21 B.G.:

20 B.G.:

16 B.G.:

c. 15 B.G.:

15 - 10 B.G.:

10 B.G.:

10 B.G. - 7 B.G.:

7 B.G.:

6 B.G.:

5 B.G.:

4 B.G.:

4 - 0 B.G.:

0 B.G.: (the events of FF7)

SIDE QUESTS: (These can happen at any point in the game providing the player has the ability to do them. For example, you will not be able to meet Lucrecia in the waterfall cave if you never found Vincent in the first place!)

0 - 2 A.G.:

2 A.G.: (the events of Advent Children)

2 - 3 A.G.

3 A.G. (the events of Dirge of Cerberus)

500 A.G.: