Keep a file in which you note the dates of your characters’ births and any particularly relevant events in their lives, such as their marriages, or the deaths of family members. A file or chart of names and dates helps you orient yourself consistently, so that you don’t inadvertently refer to one event in 1987 as having taken place when the narrator was five, and another in the same year as having taken place when he was seven.
Also keep track of the dates in the current time frame of your story. If it is spring one week, even with climate change it is unlikely to be mid-winter the next. If your character’s sister breaks her leg at Thanksgiving, the healing process will probably extend into any Christmas scenes you may want to depict.
Especially with supporting characters, authors can lose track of time and place, and errors can be overlooked by editors as well. Sharp readers will be distracted from the story by such mistakes, and after they do it takes time to get their concentration back again.