Date uses the Proleptic Gregorian Calendar, so it assumes the Gregorian leap year calculations for its entire length. As per ISO 8601, it treats 1 B.C. as year 0, i.e. 1 B.C. is 0, 2 B.C. is -1, etc. Use yearBC to use B.C. as a positive integer with 1 B.C. being the year prior to 1 A.D.

DateTime fromISOExtString(string str) {
    if(str.endsWith("60")) {
        return DateTime.fromISOExtString(str.chomp("60") ~ "59") + 1.seconds;
    } else {
        return DateTime.fromISOExtString(str);
    }
}