Elementary arithmetic, with brief notices of its history... by Robert Potts.

ON TIIE DIVISIONS AND MEASURES OF TIME. 17
A.u.O. It will be obvious this ancient mode was different from the
modern mode of reckoning. The year before, after the Roman
manner of reckoning, being considered the year of the birth, the
preceding year was reckoned the second year before the event. It
will be obvious, as the date A.D. did not begin to be reckoned from
0, but from 1, in calculating any number of days between any
year before and any year after the epoch of the Christian era, one
year must be subtracted from the sum of the dates.
The pontificate of Gregory XIII. was distinguished by the reformation of the calendar, but was indelibly disgraced by his
approbation of the massacre of the Protestants in France on St.
Bartholomew's Day in the year 1572. His infamy is perpetuated
by the medal he had struck to commemorate that deed of bloodshed.1
The eleven minutes' excess of each Julian year above the tropical
year had so accumulated between the years 325 and 1582, A.D., that
the reckoning of the vernal equinox took place 10 days too early. At
the time of the Council of Niceca in 325, the vernal equinox fell on
21st March, and the Council decreed that the festival of Easter
should be celebrated on the first Sunday after the full moon that
happened after this equinox. In 1582 the vernal equinox, according
to the reckoning, fell on 11th March; and the festival of Easter,
coming 10 days too early, occasioned also irregularity in the times of
celebrating other festivals of the Church.
In order to bring back the reckoning to the true time of the
equinox, Pope Gregory XIII. caused ten whole days to be omitted
or left out of the month of October, in the year 1582, so as to make
the years in future to agree with the seasons, and the vernal equinox
to be reckoned at the right time. And in order to prevent the recurrence of this error at any future time, it was fixed that, besides the
Julian correction of every fourth year being a bissextile, the last year
A.D. of every complete century, if divisible by 400, should also be a
bissextile year, but if not so divisible, a common year.
According to this rule the year 1600 would be a bissextile year,
and the years 1700, 1800, 1900 would be reckoned as common years.
1 When the French ambassador waited on Queen Elizabeth to report such an
account of the proceedings at Paris as Charles IX. deemed proper to be announced,
<n entering the palace he found the apartments leading to the Queen's presencechamber filled with crowds of courtiers in deep mourning, and as he passed through
them, no one offered any friendly salutation. He said that his master had instructed
him to inform the Queen that Admiral Coligny, smitten by his conscience, had
confessed to the King that the Protestants had conspired to seize both him and the
Queen mother, and that the dread of a civil war had driven his sovereign to allow
the opposite party to proceed to the execution of their enemies. On hearing his
message, the Queen expressed her esteem for King Charles, and proceeded to say:"Although, indeed, if the information had been found true, yet the manner of
cruelty used could not be allowed in any government, and least in that place, where
the King might, by order of justice, have done due execution, both on the Admiral
and on all others that should have been proved offenders. For it could not be denied
but the same force that murdered so many multitudes might more easily have put
down the leaders, especially the wounded Admiral, under arrest. She was willing
to believe that those councillors, whose age and experience ought to have made
them useful guides to their young sovereign, were more to be blamed than he."
And the Queen concluded by saying that "she did surely persuade herself that if
the King would not use his power to make some amends for so much blood, so
horribly shed, God, who saw the hearts of all, as well of princes as of others, would
show His justice, in time and place; when His honour should therein be glorified,
<s the author of all justice, and the avenger of all blood-shedding of the innocent."