We’ll probably never know the true answer to this question. However, there are some reasons I can think of that he would have done so.
1. He was writing fiction and didn’t care that much.
The narrator of the story insists that the events actually happened. Therefore, the story is supposed to be a “journalistic” piece of work, with all facts as being independently verifiable. Except that they really aren’t. There is no way a journalist could have known the exact dialog between the characters, or known what went on behind closed doors. The story is a work of fiction, and although it has overtones of journalism, the author wasn’t marketing the story as such when it was published.
2. He was afraid someone would declare a specific date and criticize that other elements didn’t match up.
There are enough elements in the story to make an educated guess as to when the story took place. However, if you were to choose a specific date, and knew other things about that date, you may come back and say other elements didn’t match up. An example would be which operas were being performed. Faust is big in the story, but I don’t know if it was really played at the Opera that much. Or, the managers – they were fictional characters, not real people.
3. The real life elements he based the story on won’t line up to a specific year.
While I don’t want to look them up for this analysis and cloud my thinking by refreshing myself on the details, I know that there have been those who have looked into historical basis for the story. There was a singer that Christine was based on, as well as a story about a counterweight falling from the chandelier. Leroux embellished these stories to create his own characters and events. The whole chandelier falling is much more dramatic than a single counterweight plunging to the floor below. It seems much more sinister when Erik does this.
Again, I don’t remember the dates, but it’s quite possible that the singer sung in Paris 15 years before the counterweight fell – or didn’t sing in Paris at all. By providing enough dates and associated details that would allow the date to be determined accurately, these other historical details would also have to fit exactly. As anyone who has written fanfic extensively knows, it can cramp your creativity to have to fit your story to the cannon of the original, or the historical facts in this case.
As an aside, I used to write CSI Fanfiction and one of the reasons I stopped was that events happened on the show that didn’t fit with the flow of events and details in the ongoing storyline I was writing.
4. The character of Erik is mysterious, and so he wanted the setting, including date, to be mysterious as well.
Leroux writes most of the story in the third person. In the third person part of the story, he takes us into Raoul’s thinking and feelings. When he switches to first person for the Persian’s narrative, he tells us what the Persian was thinking and feeling.
Interestingly, he doesn’t ever put us in Christine or Erik’s heads. Raoul, Christine, and the Persian tell us what Christine and Erik said, but only have their words to go on. We don’t get a look from inside their head to see what they are leaving out, or what feelings they are hiding. For instance, Christine never says that she loves Erik, but what she doesn’t say, and her actions, declare that she does.
Since the main players, Erik and Christine, as well as the fictional elements of Erik building the Opera house, and his house within the walls, are all mysterious and a touch unbelievable, leaving the date of the story matches perfectly.
5. Since the original story ran in installments, continuity may have suffered as a result.
It’s possible that Leroux wrote the story in installments. It’s quite likely that even if he wrote it all together, that each installment had independent editing for the paper. This can throw of continuity, if details aren’t kept closely in mind.
One detail that pops into my mind that is probably a casualty of this is Raoul’s age. In the beginning, it is stated that Phillipe is exactly 41 and Raoul is 20 years younger, therefore putting him at 21. Then, later, when Leroux is letting us into the back and forth feelings of Raoul when he gets the letter from Christine, the narrator tells us that Raoul was 20.
Other dates that don’t seem to quite add up are the ages of Christine and Raoul over the years. Again, at the beginning, Leroux tells us that Raoul’s father died when he was 12. His sisters, and later an aunt raise him.
Then, when Christine and Raoul’s past is being filled in, it’s explained that Raoul was at the shore with his aunt. Perhaps this was a time prior to his father’s death, but it would seem that it was during the time his aunt was raising him. Yet, Raoul is described as a “little boy.” Twelve isn’t a little boy, especially in a time when people married young. Then, when they met again 3 years later, they had become young adults. Fifteen seems right for being a young adult.
Did Leroux forget the age his father died from the description at the beginning of the story? He certainly seems that he forgot Raoul’s age.
Oddly, these discrepancies aren’t corrected when the story went to press as a book, instead of a series of installments. Perhaps it was thought that since each installment would have received individual editing, an overall editing wasn’t needed.
Whether that is the case or not, it is certainly reasonable to think that long term editing could have contributed to the timing discrepancies that make the exact year vague.
No matter what the reason Leroux made the story vague, it certainly adds a bit of mystery to the story. He did provide enough to give us a likely date, therefore giving us a reference for behaviors and customs of the time period. Since these factor into the story more so that exact timing, it’s a good thing that he does give us some direction as to when the story occurred.