The relative class years of Tom Riddle and Rubeus Hagrid are spelled out quite clearly. Tom Riddle entered Hogwarts two years before Hagrid did. Here's the evidence for that:
- In PS chapter 4 Hagrid said he was expelled in his third year at Hogwarts. (also CoS 7)
- In CoS13 Tom Riddle's diary says the chamber was opened "in my fifth year."
- Hagrid was expelled when he was framed for opening the Chamber of Secrets. (CoS 17)
The next step in assigning Tom and Hagrid Hogwarts entry dates seems equally clear. The events above are repeatedly referred to a happening "fifty years ago." Surely, calculating their start dates would be a very easy bit of math?
Unfortunately, it's not quite that simple. Some readers have said that they think "fifty years ago" is just a rough approximation and not anything solid enough to base exact dates on. I agree that this is a valid concern. People often don't mean phrases such as "fifty years ago" or "one hundred years ago" to be taken literally. For example, somebody who asks "What did people wear a hundred years ago?" is unlikely to mean exactly one hundred years ago on the dot. So I'll go over some of the individual cases where characters declare that events happened "fifty years ago."
|Text:||In CoS 18 Albus Dumbledore said "I taught him myself, fifty years ago, at Hogwarts."|
|Commentary:||This gives us a huge range of dates, since Tom Riddle was a Hogwarts student for seven years. Clearly, it's not meant to refer to an exact date or year.|
|Text:||In CoS 12 Draco Malfoy complained that "... Father won't tell me anything about the last time the Chamber was opened either. Of course, it was fifty years ago, so it was before his time...." Draco said this on December 25th, 1992. So if Draco meant exactly fifty years ago to the day, he would be talking about December 25th, 1942.|
|Commentary:||Draco himself said that he had big gaps in his knowledge. It was also second hand knowledge. I think that this could easily have been a very vague and approximate statement of time.|
|Text:||When Ron Weasley saw the name on the diary during CoS 13, he remembered an award he had cleaned during detention earlier in the year and said: "I know that name. . . . T. M. Riddle got an award for special services to the school fifty years ago." Ron said this during January of 1993, so fifty years ago would be 1943. (The detention day when he cleaned the award would have been September 5th of 1992 and was described in CoS 7.)|
|Commentary:||When Ron described his detention he said he had had to clean the award "about fifty times," but when he said that Riddle got the award "fifty years ago" he didn't say "about" or use any similar qualifier. This makes me think that Ron meant exactly fifty years. Also Ron saw the award himself, so this is a firsthand account. However, Ron was talking about an award he had seen months ago, so his recollection might not have been accurate. (Ron's own explanation for why he remembered the award shield clearly was that he had had to polish it many, many times.)|
|Text:||In CoS 13 (when Harry looked at the diary) "the faded year on the cover told him it was fifty years old." This scene occurred some time in January 1993, while Hermione was in the hospital wing but after lessons started up again. Therefore, exactly fifty years old would mean the year on the cover was 1943.|
|Commentary:||Harry saw an exact year, and saw it first hand. There's no vagueness about it. On the other hand it's still possible (though much less likely) that he could be speaking in general terms and rounding the years.|
|Text:||During CoS 13 Harry showed Hermione the diary "at the beginning of February" in 1993. After "looking at it closely" she said, "And this diary is fifty years old." As in the previous example, 1993-50=1943.|
|Commentary:||This example is similar to the previous one because Hermione was seeing the actual year on the book as she said this. However, I find it even more convincing than Harry's report because Hermione is repeatedly shown as a character who is a stickler for detail. I believe Hermione would be very unlikely to round the number of years off. Even if every other quote of "fifty years" was meant in a general way, this scene would convince me that the year on the diary was literally fifty years ago.|
There are several more examples of characters saying something was "fifty years ago."1 However, the examples above (especially the last two) convinced me that readers can treat these "fifty years" as an exact number of years and not just an approximation.
#1 Here are the other times characters said "fifty years." In CoS 14: Harry mentioned that Hagrid was "expelled fifty years ago." In CoS 17: The diary version of Tom Riddle said that he was "preserved in a diary for fifty years." CoS 13: Hermione said "at the beginning of February" 1993, "We know the person who opened the Chamber last time was expelled fifty years ago. We know T. M. Riddle got an award for special services to the school fifty years ago."