Old Style

Definition

Old Style, often abbreviated OS, is a label for dates recorded under the Julian calendar after some countries had adopted the Gregorian calendar, commonly called New Style. It appears especially in historical records, biographies, and older astrological works. Because calendar reform occurred at different times in different places, an Old Style date cannot be converted accurately without knowing the year and location.

Astrological Meaning

In astrology, the label matters when erecting a natal chart or timing a historical event. The calendar date must first be translated into its correct Gregorian equivalent for use with many modern ephemerides and chart programs, while preserving the recorded local time and place. The required shift is not always thirteen days; it varies by century, and regional adoption dates can create additional complications. Old Style describes the calendar notation, not a distinct zodiac or interpretive school.

Why It Matters

Recognizing Old Style helps prevent a chart from being calculated for the wrong day, which can alter the Moon’s position, house cusps, angles, and occasionally other planetary placements. It is particularly important in research involving historical figures or regions that retained the Julian calendar longer. Checking the source’s calendar convention, civil date, location, and time standard provides a more dependable chart foundation before interpretation begins.

Related Encyclopedia Entries

• Traditional Astrology
• Astrological Terminology
• Natal Chart
• Ephemeris