One-Page Checkout [Support Thread]

[lat9]

I answered this in this post, Android is mine and I have co-worker with iOS, both showing same issue.

I've gone to the test site (https://vinosdefrutastropicales.com/zc157_bs4) using my sister's iPhone, added an item to the cart, went to guest-checkout and the DOB (05/31/1970) was accepted.

I have no idea why your devices' entry is different, but if you'll do the same as above with the devices that are not accepting the date-of-birth that will help me 'see' what is going on.