Date fields offset by timezone by default. In the case of birth dates you shouldn't offset. You can disable the offset within Parameters > Display while editing your date field in CB > Field Management.
Someone signs up with a date of September 6, 90
That's their localized date, which is offset by their offset or by server offset.
In the database will be shown : 5 September 90
That's the UTC date. Date fields always store as UTC. They store as UTC and display as local.