About the Shire calendar and Gregorian calendar, Appendix D of The Lord of the Rings says as follows.
Every fourth year, except in the last year of the century, there were four Lithedays.
Mid-year’s Day was intended to correspond as nearly as possible to the summer solstice.
The Shire dates were actually in advance of ours by some ten days, and our New Year’s Day corresponded more or less to the Shire January 9.
2. is inconsistent with 3. From 1950 AD to 2050. If our New Year’s Day is corresponded to the Shire January 9, Mid-year’s Day never corresponds to the summer solstice. If our New Year’s Day is corresponded to the Shire January 10, in about half of years Mid-year’s Day corresponds to the summer solstice. But it is not preferable, because Leap-year is based on our century. So, Every fourth year of 1. is ignored. And 2. is adapted as follows.
Calculate days from the summer solstice to next the summer solstice.
If it is 366 days, this year is Leap-year and there is the Overlithe.
From above Gregorian calendar is corresponded to the Shire calendar.
In this procedure, there are five Leap-years in every 25 years. Once a 25 years, there are five years between two Leap-years. Mid-year’s Day must correspond to the summer solstice.
Mid-year’s Days and days to next Mid-year’s Days are shown in table A Mid-year’s Day of years from AD 1950 to 2056 and corespondence of calendars. (Mid-year’s Days to 2000 AD are from Koyomi Benrichô Edited by Koyomi keisan kenkyûkai. For after 2001, it is calculated.)
For years from 1950 to 2056, five tables are made to correspond calendars. For first half of year, table B, table C, and table D are made. For second half of the year, table E and table F are made.