Negative Time Difference Solution
For the preprocessing of the mths_since_earliest_cr_line, isn't it possible to filter for the negative values and make the earliest_cr_line_date strings? This way we can replace the first two digits of the year and make them "19xx" instead of "20xx". Then we could make them timestamps again and be able to use them correctly. This is a solution I came up with, although would like to know if it would have any unintended consequences.