LYCOS RETRIEVER 
Julian Calendar: Pope Gregory Xiii
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[T]he Julian calendar year eventually proved to be slightly longer than the solar year. By the 16th century, the beginning of spring fell in early March. Pope Gregory XIII, acting on the advice of Christopher Clavius, therefore excised 10 days from the calendar by shortening October 1582, and he revised the leap-year rule: No leap years in centesimal years (e.g. 1700, 1800, 1900), except those divisible by 400 (e.g. 2000, 2400, 2800).
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In 1582, Pope Gregory XIII adjusted the Julian calendar to correct for its inaccuracy of slightly more than 11 minutes per year. As a result, the day following October 4, 1582 was October 15, 1582. CALDAT follows this convention, as illustrated by the following commands:
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...Pope Gregory XIII eliminated the 11 minute, 14 second error in the Julian calendar by giving February an extra day in certain century years. The first two of those leap centuries were 1600 and 2000....
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Pope Gregory Xiii