Different dates
Definitions
Because I’ll mention day and date quite a bit, let me start with some brief definitions. To keep things simple, I intentionally ignore time zones, as they typically don’t matter for calendars.
Day
A twenty-four-hour period starting and ending at midnight, distinct from the ones before and after it. Each day has exactly one predecessor, yesterday, and one successor, tomorrow. Days are sequential and do not have inherent names.
Date
A calendar-based label assigned to a specific day. Dates and days have a one-to-one relationship but are distinct. A date labels a day within a specific calendar system. A single day can have different dates across calendars.
Underspecified Date
A date with at least one component missing, typically the year. It represents a repeating pattern within a specific calendar system rather than a single, unique day.
For example, the signing of the Declaration of Independence happened on a specific day. In the Gregorian Calendar, this day is labeled with the date “4th July 1776”. When people celebrate Independence Day, they typically do so on the “4th of July”, which is an underspecified date. It represents an annually repeating pattern on the Gregorian Calendar and doesn’t refer to a specific day.
The same applies to your birthday. Technically, you only have one “birth day”.
Calendars
However, dates don’t have to be Gregorian Calendar dates. There are many different calendars and each day has a corresponding, and typically different, date on those calendars. Calendars help us refer to specific days because without them days can only be referenced relative to other days or events, like “in ten days”, “the day after the accident”, “five days ago”, etc. In its simplest form a calendar can even be something trivial like counting days. The most prominent example is the Julian Day Number (JDN).
| JDN | Event |
|---|---|
| 2369916 | Signing of the Declaration of Independence |
| 2416466 | First Human Flight (Wright Brothers) |
| 2434531 | Coronation of Queen Elizabeth II |
| 2440423 | First Moon Landing |
| 2447840 | Fall of the Berlin Wall |
| 2449470 | End of Apartheid in South Africa |
| 2454281 | Release of the First iPhone |
| 2454687 | Beijing Olympics Opening Ceremony 2008 |
Most calendars, however, use one or more subdivisions like, years, seasons, quarters, months, and weeks to be more useful than just an ever-increasing number.
Different dates on different calendars
Switching to SAC13, or any different calendar system, will inevitably lead to different dates referring to the same day. For example the signing of the Declaration of Independence happened on 4th July 1776 in the Gregorian Calendar, and on 23rd June L776 in SAC13. Because a year doesn’t always have the same length (365 or 366 days) and SAC13 has a different leap year rule, the underspecified dates 4th July Gregorian Calendar and 23rd June SAC13 don’t always land on the same day for a given year.
| Gregorian | SAC13 |
|---|---|
| 4th July 2023 | 22nd June M023 |
| 5th July 2023 | 23rd June M023 |
| 4th July 2024 | 23rd June M024 |
| 4th July 2025 | 23rd June M025 |
| 4th July 2026 | 22nd June M026 |
| 5th July 2026 | 23rd June M026 |
For other celebrations that are tied to specific weekdays, like, for example, Mother’s Day and Easter, it’s probably simplest to always celebrate them on the same day in SAC13 because SAC13 has stable weekdays. For Mother’s Day, 28th April in SAC13 would be a good candidate and would always land on the 7th weekday.
If you want to test other days like your birthday, anniversary, holidays, etc., you can check out the calculator.