Writers Cannot Do Math: Wendy tells the doctor that Danny's shoulder had been dislocated six months earlier. Later, while bitching to Lloyd the Bartender, Jack says that the dislocated shoulder incident had happened three years earlier. This becomes some very creepy Fridge Horror if you think about how Wendy specifically phrased it: the time of the accident isn't specified, only that Jack immediately afterward promised not to have another drink in case he hurt her or Danny again. So there's a period of two and a half years during which Jack must have done something to break his promise, which Wendy clearly doesn't want to tell the doctor about.
Jack also mentions that he hasn't had a drink of alcohol in FIVE months, which seems to indicate that there was about a month in between the time where Jack promised to stop drinking and the incident with Danny. Implies a good few weeks of uncomfortable nights in the Torrance household, if nothing else.