BC stands for "before Christ" and refers to years before Jesus's birth. BC dates start from 1 BC and count backward. BC dates are written with "BC" after the year (e.g., 1200 BC). BC dates rarely extend beyond 10,000 BC except in science
Pope Gregory XIII introduced the Gregorian calendar in 1582 to correct the Julian calendar's discrepancy. The Gregorian calendar is based on the Julian calendar's 365.2425-day average. Britain switched to the Gregorian calendar in 1752, while the US followed in 1752
Epoch is a chosen time point as origin of a calendar era. Epoch serves as reference point for time measurement. Epoch moment usually determined by congruity or conventions
Calendars fall into four categories: lunisolar, solar, lunar, and seasonal. Most pre-modern calendars are lunisolar. Islamic and some Buddhist calendars are lunar. Modern calendars are solar, based on Julian or Gregorian systems
Era is a time span defined for chronology or historiography. Term "era" entered English in 1615 from Latin "era" meaning "time reckoning". First appeared in Visigothic Spain in 5th century
Century is a period of 100 years or 10 decades. Word comes from Latin centum meaning one hundred. Century is abbreviated as c