Dr. Watson provides a specific date for the beginning of the events recorded in "The Adventure of the Speckled Band."
It was early in April in the year '83 that I woke one morning to find Sherlock Holmes standing, fully dressed, by the side of my bed.
Watson also states that he had been studying Holmes' methods for eight years.
ON GLANCING OVER my notes of the seventy odd cases in which I have during the last eight years studied the methods of my friend Sherlock Holmes, I find many tragic, some comic, a large number merely strange, but none commonplace; for,working as he did rather for the love of his art than for the acquirement of wealth, he refused to associate himself with any investigation which did not tend towards the unusual, and even the fantastic.
Arthur Conan Doyle introduced Sherlock Holmes in A Study in Scarlet in 1887. In that novel the narrator Dr. Watson tells some facts about his own life prior to meeting Holmes. Watson was an army surgeon stationed in India. He was badly wounded at the battle of Maiwand, which took place in July of 1880. Eventually he was sent home. He must have arrived in England in 1881, and he states that he lived for some time in a residential hotel on a government allowance of eleven shillings and sixpence per day.
So alarming did the state of my finances become, that I soon realized that I must either leave the metropolis and rusticate somewhere in the country, or that I must make a complete alteration in my style of living. Choosing the latter alternative, I began by making up my mind to leave the hotel, and to take up my quarters in some less pretentious and less expensive domicile.
So it would appear that he met Sherlock Holmes for the first time in 1882 and had been sharing rooms at Baker Street with him for only about a year when Helen Stoner came to consult Holmes about the "speckled band."
"The Adventure of the Speckled Band" was published in February, 1892, but Watson states:
It is possible that I might have placed them upon record before, but a promise of secrecy was made at the time, from which I have only been freed during the last month by the untimely death of the lady to whom the pledge was given.
Arthur Conan Doyle had a good reason for setting the story back in the past, during the time when Watson was still living at Baker Street. Doyle wanted Helen Stoner to arrive very early in the morning, and it would have been difficult to involve Watson in the case from the beginning if he were not able to join Holmes in the initial interview. The first part of the story consists of a long, detailed account of the complicated family history, including a description of Julia Stoner's death two years earlier. Dr. Watson would have had to hear all of this exposition at the time, because there would have been no way for Doyle to repeat it later on. As it is, Helen Stoner's narrative seems excessively long. It is a story within a story. Holmes can say very little except occasionally to interject comments such as
“I am all attention, madam.”
“Your sister is dead, then?”
Readers in Victorian times were more patient than they are today. Doyle had a lot of background exposition to convey, and he chose to get it all out of the way at once. The author does something very similar in "The Red-Headed League" where he has Jabez Wilson tell a long story about how he came to be employed by that strange league and how he lost his job after working faithfully at copying from the Encyclopedia Britannica for eight weeks.