Convert real-world dates into the Forgotten Realms (Faerûn) calendar format.
Inspired by Dungeons & Dragons and the Harptos calendar.

faerun-date is a JavaScript library that transforms standard dates into the calendar system of Faerûn.
It includes months, 10-day weeks (tendays), seasonal cycles, and Faerûnian holidays like Greengrass and Midsummer.

⚠️ Note: This is a simplified model of the Harptos calendar. All months are fixed at 30 days, holidays are added to the count, and leap years are every 4 years (Shieldmeet). Day names are placeholders, not canonical.

• 📅 Converts real-world dates to Faerûn calendar dates
• 🗓️ Supports Faerûn-specific months and festivals
• 🌱 Seasonal calculation (Spring, Summer, Autumn, Winter)
• 📆 Week number support (Faerûn “tendays”)
• 🕊️ Leap years with Shieldmeet (every 4 years, simplified)

npm install faerun-date
# or
bun add faerun-date

import FaerunDate from 'faerun-date';

const date = new Date(1492, 4, 5); // 5 Mirtul 1492 (months are 0-based)
const faeDate = new FaerunDate(date, { faerunYear: 1492 });

console.log(faeDate.toLocaleString());
// Example: "Far, 05 Mirtul 1492 DR – Season: Spring – Week 19"

console.log(faeDate.getWeekOfYear());
// 19

Creates a new Faerûn date from a real-world Date.
Options: { faerunYear?: number }

Returns a human-readable string:

Far, 05 Mirtul 1492 DR – Season: Spring – Week 19

Returns the name of the current day in the 10-day week cycle:
Sul, Far, Tar, Sar, Rai, Zor, Kyth, Hamar, Ith, Alt.

Note: These labels are not canon; they represent tendays.

Returns the Faerûnian month name.

Returns the season:
Winter, Spring, Summer, or Autumn.

Returns the current year in Dale Reckoning (DR), if provided via options.
Otherwise returns null.

Returns the name of the festival (Greengrass, Midwinter, etc.)
if the date matches one, otherwise null.

Returns the week (tenday) number of the year.

Returns a calendar-style string:

• [Festival] Greengrass if the date is a festival
• 05 Mirtul otherwise

Returns true if divisible by 4.

Constructs a FaerunDate from a date string.

Shortcut to .toLocaleString().

This package includes a CLI tool to compare Gregorian weeks with Faerûn tendays.

npx faerun-compare-weeks --year 1489 --weeks 10

Example output:

Gregorian Week → Faerûn Tenday Correspondence for year 1489
Week 01 → Tenday 01 (Mon Jan 01 1489)
Week 02 → Tenday 02 (Mon Jan 08 1489)
...

• --year <YYYY> – Starting Gregorian year (default: current)
• --weeks <N> – Number of weeks to display (default: 20)

Note: Gregorian weeks in the CLI are calculated with a simplified formula (new Date(year, 0, (week-1)*7+1)); not ISO‑8601.

• All months are fixed at 30 days (simplification).
• Leap years are every 4 years (Shieldmeet).
• Day names are placeholders, not canon.
• Faerûn year (DR) must be passed manually in options.
• Some lore-specific details of Harptos are not modeled.

Released under the MIT License.
© 2025 Cantilux