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From time to time, countries might change their time zones by an hour or a multiple of 15 minutes for various reasons. Some countries, however, go to great extremes of changing on which side of the land the International Date Line passes through.
If you look at the 16th Philippine colony history, that country was using a calendar that was one day off in relation to the calendars other countries in the same time zone were using. Example. If a country north or south of the Philippines were using a Julian calendar dated May the 5th, the Philippines were using a Julian calendar dated May the 4th, as well as a weekday one day earlier than the weekday the other countries in the same time zone were using. It had to do with keeping the date in sync with the colonies in the Western hemisphere, though the Philippines lie in the Eastern hemisphere.
This also complicates the assignemnt of the Julian Period Date for the Philippines. A rule is that the Julian Period Date must be uniform in relation to the time zone offset that it lies in, and in this case, UTC+8:00, which is the normal offset. The Philippines, by using an offset of UTC-16:00, was using a calendar a full 24 hours off. To align the date with the Julian Period Date, we invented a suffix to attatch to the version of the calendar that the country was using.
So in the 16th through the 19th centuries, the Philippines used three versions of the Julian calendar as ordered by Spain, the colonizer at the time. Under Spain's rule, it used the Julian-CD+-1D calendar, followed by the Julian-1-1D calendar, and finally, the Gregorian-1-1D calendar. The -1D is used to indicate that this calendar is 24 hours slow in relation to the time zone.
So let's first get a Julian Period Day for a Gregorian-1 calendar day. Let's choose March 15, 1844, the last year the Philippines were using an offset in their country. The Julian Period Day would start at 2394640.5 at midnight for that day on that calendar. Now, let's use that Julian Period Day for the Gregorian-1-1D calendar. It's easy. Just subtract one day from the day on the Gregorian-1 calendar, and you have March 14, 1844, Gregorian-1-1D calendar date, corresponding to the Julian Period Day of 2394640.5. Simple.
Calculator used: https://aa.usno.navy.mil/data/JulianDate
On the first day of 1845, the Philippines dropped December 31, 1844 to line up their calendar with the calendars used in their time zone. There was a December 31, 1844 on the Gregorian-1 calendar, which matched the date of December 30, 1844 on the Gregorian-1-1D calendar. The Julian Period Day would be 2394931.5 at midnight. When January 1, 1845 arrived, the Julian Period Day was 2394932.5 at midnight. That Julian Period Day matched December 31, 1844 on the Gregorian-1-1D calendar, which was discontinued by the Philippines at the end of the previous day, so the Philippines simply skipped a day by switching versions of the Gregorian-1 calendar from going a day behind with the Gregorian-1-1D calendar to the Gregorian-1 calendar.
On the other extreme, some countries just east of the International Date Line decided to keep in sync with countries to its West by moving the International Date Line to run past the Eastern end of their country, therefore, not only synching up with countries or lands they are doing business with, but at the same time, falling out of sync by one day by advancing their dates fast one day. In order to achieve the effect of moving from East of the IDL to West of the IDL, they have to skip a day. Once again, the Julian Period Day comes into play as that should never be changed in either direction.
In 1995, the country of Kiribati wanted the whole country to no longer be divided by the International Date Line. So that year, Kiribati switched from the Gregorian-1 calendar to the Gregorian-1+1D calendar, where it was one day fast in relation of the Hawaiian Islands to its north. Where it was still December 31, 1994 for the countries in the UTC-10 offset time zone offset, it was January 1, 1995 for the country in the proclaimed UTC+14:00 offset. The Julian Period Day for this day, December 31, 1994, Gregorian-1 date or January 1, 1995, Gregorian-1+1D day, was 2449717.5 at midnight. The day after, January 1, 1995, Gr-1, or January 2, 1995, Gr-1+1D, the JPD was 2449718.5 at midnight. A country can move the IDL to the east or west, but above all, the Julian Period Day must be preserved for time chronology purposes.
The state of Alaska was once a territory of the Russian empire, and while it was a part of the Russian empire, its offset was UTC+13:00, UTC+14:00, or UTC+15:00 assuming that three Alaska time zones existed then. The International Date Line once passed through the Eastern side of Alaska. Alaska used a Julian-1+1D calendar beginning in 1700 to sync up with the Russian empire so that all of the Russian properties lie in the same date.
The Russian properties west of the traditional IDL that roughly followed the 180 degree meridian used the Julian-1 calendar with UTC offsets ranging from +2:00 to +12:00 with Alaska using time zones ranging from +13:00 to +15:00.
In 1867, Russia sold the territory of Alaska to the U.S.A., which was using the Gregorian-1 calendar. This made Alaska the property of the U.S.A. This also made Alaska, which was using the Julian-1+1D calendar, 23 hours ahead of the states or territories of California, Oregon, and Washington, as they used the offset of UTC-8:00, while the eastern end of Alaska used UTC+15:00 instead of UTC-9:00 as islands and countries were using along that time zone offset.
To correct that, effective in October of 1867, Alaska would switch from the Julian-1+1D calendar, to the Gregorian-1 calendar. The effect of the switch would result in Alaska dropping 11 days (instead of 12 because of the date offset of the calendar change), as well as repeating a weekday, in this case, a Friday. Also, the International Date Line would be placed around the 180 meridian to the territory's west side, splitting Russia and Alaska by a day.
Let's walk through this process.
Calendar used: October 1867 Julian/Gregorian: Sun 1-Fri 6, then Fri 18-Thu 31 (loses 11 days and repeats one weekday.)
| JPD | Julian-1 Date | Julian-1+1D Date | Gregorian-1 Date | Date in Alaska |
|---|---|---|---|---|
| 2403252.5 | Sun Oct 1 | Mon Oct 2 | Sun Oct 13 | Mon Oct 2 |
| 2403253.5 | Mon Oct 2 | Tue Oct 3 | Mon Oct 14 | Tue Oct 3 |
| 2403254.5 | Tue Oct 3 | Wed Oct 4 | Tue Oct 15 | Wed Oct 4 |
| 2403255.5 | Wed Oct 4 | Thu Oct 5 | Wed Oct 16 | Thu Oct 5 |
| 2403256.5 | Thu Oct 5 | Fri Oct 6 | Thu Oct 17 | Fri Oct 6 |
| 2403257.5 | Fri Oct 6 | Sat Oct 7 | Fri Oct 18 | Fri Oct 18 |
| 2403258.5 | Sat Oct 7 | Sun Oct 8 | Sat Oct 19 | Sat Oct 19 |
| 2403259.5 | Sun Oct 8 | Mon Oct 9 | Sun Oct 20 | Sun Oct 20 |
As you can see, Alaska had an eight-day week when they switched calendars from one being one day fast to one being aligned with the timezone offset, going from UTC+15:00 to UTC-9:00. They were just one hour behind the Western states which sat at UTC-8:00.
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