An object usually believed to be a Celtic example of these things was unearthed in Coligny, France. 
An object usually believed to be a Celtic example of these things was unearthed in Coligny, France. After Chesterfield's Act implemented one of these things, a mob chanting the number eleven rioted in London. One of these things was supported by the papal bull Inter Gravissimas. Charles-Gilbert Romme developed one of these things that included Pluviose, Prairial, and Fructidor. A Roman example of these things named for Numa Pompilius was replaced by one named for Julius Caesar that was used until 1582. For 10 points, name these systems, one of which is named for Gregory XIII and is divided into twelve months.
An object usually believed to be a Celtic example of these things was unearthed in Coligny, France.
calendars [or Gregorian calendar; or Julian calendar; or French Revolutionary calendar]