Explanation: Shoplifters are people who steal from a store, usually something small (but shoplifters also steal larger items). Shoplifting is against the law so, if you are caught by the police, you might have to go to jail. If you are caught by the police you will have a trial and get sentenced to spend time in jail. In this case, “He got [sentenced to] 12 months” means that he will spend 12 months in jail. This joke is funny because a calendar also has 12 months, and that is what he got when he stole the calendar.