Kyoya was an unplanned child. If you subtract 37-40 weeks from his birthday, he was made during valentine's day.
This is further supported by the fact all his siblings are significantly older than him.
Like I did some math, Yuuichi is probably 11-12 years older than him. Fuuymi based on canon is 9-10 years. Akito just gives complete middle child syndrome and is probably 6-8 years older than kyoya.