This equation gives the dominical letter of any year from the commencement
of the era to the Reformation. In order to adapt it to the Gregorian
calendar, we must first add the 10 days that were left out of the year
1582; in the second place we must add one day for every century that has
elapsed since 1600, in consequence of the secular suppression of the
intercalary day; and lastly we must deduct the units contained in a fourth
of the same number, because every fourth centesimal year is still a leap
year. Denoting, therefore, the number of the century (or the date after the
two right-hand digits have been struck out) by c, the value of L must be
increased by 10 + (c - 16) - ((c - 16) / 4)_w . We have then