> Because of the Gregorian Calendar Reform of 1582, the numbers of days after 1001-01-01 are fictitious, strictly spoken. They are valid only then, if we adopt the reform for dates before 1582, subsequently.
The explanation is muddled. This concept has a name, the definition at WP is succinct and clear: http://enwp.org/Proleptic_calendar
> Because of the Gregorian Calendar Reform of 1582, the numbers of days after 1001-01-01 are fictitious, strictly spoken. They are valid only then, if we adopt the reform for dates before 1582, subsequently.
The explanation is muddled. This concept has a name, the definition at WP is succinct and clear: http://enwp.org/Proleptic_calendar