The Julian Calendar, adopted in 46BC by Julius Caesar, adds one day every

four years to correct for the fact that Earth's solar year is slightly more

than 365 of Earth's daily rotations. ( 365.25 days per year)

The Gregorian Calendar, adopted in 1582 by Pope Gregory XIII, ordered that

leap years should not occur in years ending in '00', unless divisible by

400. ( 365.2475 days per year)

A modern estimate of a calendar year is 365.24219 rotations of Earth per

solar year.

Calendar days since 46BC

46BC = 366

(( 2004 + 46) * 365) = 748250

(( 2004 + 46) / 4) = 512

( 1600, 2000) = -2

= 749126

Actual days since 46BC

( 2004 + 47) * 365.24219

= 749111.73169

Accumulated error

= 14.26831

Using the Gregorian Calendar accumulates about 5 Earth rotational day error

for every thousand solar years.

An even more modern calendar error correction rule can be stated as,

add one day every four years, using February 29th, unless the year is

exactly divisible by 128. ( 365.2421875) This may accumulate 1 Earth

rotational day error for every hundred thousand solar years.