Birthday happening before calender date

okay I'm gonna try to explain this as good as I can. So I'm playing sims 4 on my xbox one x and I just got the seasons expansion  And now I see that their birthdays are on the calendar. So I plan the parties ahead so I dont miss them and make my kids sa . But one of my kids keeps on having his birthday before the calendar is saying its gonna happen. I dont know what I'm doing wrong!?!?!