---
title: "Introduction to ethiodate"
output: rmarkdown::html_vignette
vignette: >
  %\VignetteIndexEntry{Introduction to ethiodate}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

```{r, include = FALSE}
knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
)
```


[The Ethiopian calendar](https://en.wikipedia.org/wiki/Ethiopian_calendar) is an official calendar in Ethiopia and differs significantly from [the Gregorian calendar](https://en.wikipedia.org/wiki/Gregorian_calendar) system. The Ethiopian calendar is closely related to [the Coptic Egyptian calendar](https://en.wikipedia.org/wiki/Coptic_calendar), and both have 13 months: 12 months of 30 days and a 13th month (Pagumē) of 5 or 6 days, depending on the year. The word Pagumē comes from a Greek word that translates to 'forgotten days'.

The Ethiopian calendar system is 7-8 years behind the Gregorian calendar, and the new year occurs on September 11 or 12 depending on the year, whether leap or not. Unlike the Gregorian calendar system, leap years occur every four years without exception.

The `ethiodate` package provides tools for parsing, converting, and working with Ethiopian dates in R. This vignette demonstrates how to parse Ethiopian dates from strings or numeric formats and convert them into `ethdate` objects.

# The Numeric Logic of ethiodate

Just like R’s base `Date` class, the `ethdate` class represents dates as number under the hood — specifically, the number of days since a fixed origin date. While `Date` uses the Gregorian origin "1970-01-01", `ethdate` uses the Ethiopian equivalent of that date (i.e., the Ethiopian date that corresponds to Gregorian 1970-01-01) as its base. This design choice aligns the internal storage of `ethdate` with how base R handles dates, ensuring consistency and simplicity.

Underneath, an `ethdate` object is just a double with a class attribute. This means all the advantages of using number-based date arithmetic — such as speed, simplicity, and compatibility — are preserved. Adding or subtracting a number from an `ethdate` gives a new date offset by that many days. Comparing two `ethdate` objects just boils down to numeric comparisons. The internal simplicity of this representation enables intuitive and efficient date operations without complex calendar logic at the arithmetic level.

This architecture also makes the `ethdate` class easily interoperable with many parts of R's ecosystem. Since it's based on numeric, `ethdate` objects can be stored in data frames, used as keys in joins, and sorted naturally. When you convert to Gregorian using `as.Date()`, the underlying integer is interpreted correctly relative to the Gregorian calendar system. This separation of storage and presentation keeps the internals clean while allowing rich formatting and conversions when needed.

Overall, by mimicking the design of base R's `Date` class but anchoring it in the Ethiopian calendar, `ethdate` achieves both elegance and power: it’s lightweight, predictable, and optimized for calendar operations tailored to the Ethiopian context.

# Key Functions

- `eth_date()`: a generic function that casts vectors of other class to ethdate class. 
- `eth_parse_date()`: a function that parses date from strings.
- `eth_make_date()`: a function that makes a date from year, month, and day components of the date.



# A Quick Guide

- [Parsing Ethiopian Dates](parsing.html)
- [ethiodate and ggplot2](ggplot.html)

