A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
I would suggest there is nothing wrong with Excel and it doesn't need repairing - it is behaving as it always does.
What you have come up against is Excel's "helpful" date handling, that sometimes is but invariably isn't. Whenever you enter something that looks like a date in Excel (such as April 1966, 1/4/66 or similar), Excel tries to help by interpreting it as a date and formatting it accordingly. In the case of April 1966, it has changed it to 1st April 1966 (1/4/66). As far as I am aware there is no way to switch this feature off.
The 24198 is the number of days since 1/1/1900. Excel calculates dates as days, and part thereof, since 1/1/1900, thus 1/1/1900 is day 1, and 1/4/1966 is day 24198. Midday on that same day is 24198.5 and so forth.
I think the only way to genuinely be able to enter them as free format is to format the column as Text, or precede the entry with an apostrophe (') as you enter it to force it to be treated as text.
EDIT: PS - As an afterthought, if you're doing genealogy you're almost certainly going to encounter pre-1900 dates eventually, so formatting as text is really your only option without a lot of mucking around.