Generally Dutch birth and death records start by saying the registration date. The actual date is somewhere in the text. I've seen in multiple occasions that only the registration date is indexed, probably because that's the easiest to see.
Other websites that use those indexes from the archives, like WieWasWie and FamilySearch, will than falsely fill in the registration date as a birth date, by lack of an alternative. So you should always watch out whether you've found the actual birth date or the registration date (which is often within a few dates after birth).
And of course the WikiTree birth date field is for the actual birth date, not the registration date.