The Ethiopian calendar is an official calendar in Ethiopia and differs significantly from the Gregorian calendar system. The Ethiopian calendar is closely related to the Coptic Egyptian 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.
Just like R’s base Date class, the ethdate
class represents dates as integers 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 an integer with a
class attribute. This means all the advantages of using integer-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 integer 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
integers, 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.
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.