On closer examination I noticed a considerable number of contacts have the birth date 30 November 1999 and I certainly didn't enter this date into the contacts.
What I suspect has happened is I've entered contacts into my iPhone, at one point synchronised the contacts with iCloud and then the contacts have been picked up from iCloud by the MacBook Air. I've not directly entered the contacts. into the MacBook Air. I use other software for recording my contacts, but I do enter contacts into my iPhone.
It looks like instead of Apple not entering values into unpopulated fields, they've made an incorrect decision to populate the date fields with a default value.
To fix the issue the only way I've found so far, is to visit each contact and remove the birthday and anniversary fields.
As a person who has done software development in the past, it always interests me to see these quirks where someone hasn't perhaps thought things through as well as they could have. These entires were entered back on an old iPhone 3GS, so perhaps this issue only affects people who have contacts entered many years ago.
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