I don't really thing that assigning real dates to the films is appropriate. However, time spans is more easy to do. I tried to make a timeline before, based on the information provided on the films. I made Bruce's birthday on Batman Begins as a point of reference.
BB stands for Bruce's Birthday on Batman Begins.
- 30 years before BB: Bruce Wayne is Born to Martha and Thomas Wayne.
- 22 years before BB: Bruce Wayne parents are killed by Joe Chill. Chill is captured the same night of the incident. Bruce is 8 years old.
- Joe Chill serves 14 years in prison.
- Eight years before BB: Bruce Wayne initiates his journey around the world. Bruce is 22 years old.
- Bruce Wayne spends 7 years in training and finding purpose, the latter part of that time span is spent with the League of Shadows.
- One year before BB: Bruce Wayne returns to Gotham City to fight crime as Batman, with the intent of inspiring good in people. During that year he does the following: Spends some time setting up his plan, gathering his equipment, investigating the mob, and making Sgt. James Gordon an ally. Then as Batman he takes out Falcone and the Scarecrow, while uncovering the League of Shadows' plan to destroy Gotham. Bruce is 29 years old.
- BB: Wayne Manor is burned down by the League of Shadows on Bruce Wayne 30th birthday. Batman stops Ra's Al Ghul from destroying Gotham with the fear toxin by destroying the monorail. Bruce is 30 years old.
- After short unspecified amount of time after BB: Batman Begins Ending: Sgt James Gordon is promoted to Lieutenant, reveal of the Bat-signal and advent of The Joker.
Now you can say that roughly all of the main events in Batman Begins take less than a year, by taking into account Bruce's age.
The consensus is that between Batman Begins and The Dark Knight it passes from about 6 or 9 months to a year. I'm counting the viral campaign for the Dark Knight, namely the Gotham Tonight segments, as the closest source of information outside of the films since the major actors of the films participate on this.
The six Gotham Tonight segments are paced in intervals of two weeks except the last one, which airs just one week after the one that came before, making a total of 9 weeks.
Now, on the Gotham Tonight segments, is stated that between the fear toxin terrorist attack on the Narrows and the third Gotham Tonight 9 months pass. According to the airings of the episodes, 5 weeks pass between the third segment and the last (which happens at the same time of the bank robbery at the opening of The Dark Knight). So going by this, 10 months and one week pass between the attack on the Narrows and the beginning of The Dark Knight.
- 10 months and one week after BB: The Joker and his henchmen rob Gotham National Bank, setting in motion the events of The Dark Knight.
Now, the time span of The Dark Knight events could take about two or three months. The security camera showing the Joker has a time stamp saying that they were taken on July 17, 2008. It is also said that the court documents of the RICO conspiracy case confirm that this trial is also held on July of the same year, but I cannot confirm that right now. In my opinion, the events of the film could stretch at least into August.
But as I said before, I'm trying to not take into account the year, but the time span.
- Roughly a year after BB: The Dark Knight's ending: Joker is incarcerated in Arkham Asylum, Two-Face dies in a confrontation with Batman, and Batman takes the blame for Two-Face murders to ensure Gotham well being. At some point after this events, Bruce turns 31.
Now, it is said that The Dark Knight Rises is 8 years after the ending of the previous film, the last reported sighting of the Batman.
- Eight years and six months after BB: The Beginning of The Dark Knight Rises: Bane kidnaps Dr. Pavel on a CIA plane.
- Nine years after BB: Harvey Dent's Day celebration. Bruce is 39 years old.
Now, this is pure conjecture on my part, but before Bane defeats Batman and sends him to the pit, you could say it passes a month, more or less.
- Nine years and a month after BB: Bane sends Batman to the Pit.
Then the five months of Bane holding out the city passes and then, after Batman "dies" with the bomb on the ocean.
- Nine years and six months after BB: Batman saves Gotham by taking the bomb to the ocean.
Then an unspecified amount of time passes in the last flashback until the very end. You could say that Bruce is 40 years old.
This was a little difficult to do. I'm sorry, I know that taking Bruce's birthday is kind of arbitrary, but it the only important point I thought to make the timeline. I know I made some conjectures too, but feel free to discuss and complete this "timeline". If we clear this out, maybe we can incorporate other important milestones.