"I knew it wouldn't be safe to open the Chamber again while I was still at school. But I wasn't going to waste those long years I'd spent searching for it. I decided to leave behind a diary, preserving my sixteen-year-old self in its pages, so that one day, with luck, I would be able to lead another in my footsteps, and finish Salazar Slytherin's noble work."
-- Tom Riddle (CS17)
He uses murder of Myrtle in June (BLC) and the knowledge he gained from Slughorn to work the Dark Magic required to turn the diary into a Horcrux (HBP23).
Tom says that he placed his "sixteen-year-old self" into the diary (CS17), so he must have done it at some point during 1943 (he turned 16 on December 31, 1942, and turned 17 on December 31, 1943. Myrtle's murder was in June of that year and the murder of Riddle's father was in either July or August. The conversation with Slughorn about creating multiple Horcruxes took place after the summer of 1943:
Harry looked around as Dumbledore appeared beside him and saw that they were standing in Slughorn's office. Half a dozen boys were sitting around Slughorn, all on harder or lower seats than his, and all in their mid-teens. Harry recognized Voldemort at once. His was the most handsome face and he looked the most relaxed of all the boys. His right hand lay negligently upon the arm of his chair; with a jolt, Harry saw that he was wearing Marvolo's gold-and-black ring; he had already killed his father.
Rowling listed the Horcruxes and which murders were used to create them:
Lady Bella: Whose murders did Voldemort use to create each of the Horcruxes?
J.K. Rowling: The diary - Moaning Myrtle. The cup - Hepzibah Smith, the previous owner. The locket - a Muggle tramp. Nagini - Bertha Jorkins (Voldemort could use a wand once he regained a rudimentary body, as long as the victim was subdued). The diadem - an Albanian peasant. The ring - Tom Riddle snr. (BLC)