Birthday duplication gone wrong

I have two iPhones with the same Apple ID... So when I make changes to a contact on anyone iPhone, the other one gets updates automatically through iCloud.

I added a birthday for a contact on one phone and it got updated in the calendar automatically... however the duplication / auto update that should happen on the other phone records the birth date as the next day... For example, if i put July 4 as the birth date of a contact on one phone, the other phone shows July 5 as the birth date of that same contact.

Why is this happening? And how can I resolve it?