Bongabdo

A Rust library for converting Gregorian dates to Bengali calendar dates across multiple regional standards.

This crate supports:

the current official Bangladesh civil calendar
the older Bangladesh revised calendar
the traditional West Bengal solar calendar

It is designed for cases where Bengali dates differ by region and where "today" should respect the local civil day in Dhaka or Kolkata rather than a UTC day boundary.

Supported Standards

CalendarStandard::Bangladesh Current official Bangladesh civil calendar after the 2019 revision.
CalendarStandard::BangladeshLegacy Earlier Bangladesh fixed calendar adopted in 1987.
CalendarStandard::WestBengalTraditional Traditional West Bengal solar calendar modeled from sidereal solar ingress.

Installation

Add this to your Cargo.toml:

[dependencies]
bongabdo = "0.1.0"

Quick Start

use bongabdo::{Bongabdo, CalendarStandard};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let bd = Bongabdo::from_ymd(2026, 4, 14, CalendarStandard::Bangladesh)?;
    let wb = Bongabdo::from_ymd(2026, 4, 15, CalendarStandard::WestBengalTraditional)?;

    println!("Bangladesh: {}", bd.to_roman_string());
    println!("West Bengal: {}", wb.to_roman_string());

    Ok(())
}

Choosing A Standard

Use CalendarStandard::Bangladesh when you want the current official Bangladesh civic calendar.

use bongabdo::{Bongabdo, CalendarStandard};

let date = Bongabdo::from_ymd(2026, 4, 14, CalendarStandard::Bangladesh)?;
assert_eq!((date.year, date.month, date.day), (1433, 1, 1));
# Ok::<(), bongabdo::BongabdoError>(())

Use CalendarStandard::BangladeshLegacy when you need compatibility with the older Bangladesh revised calendar.

use bongabdo::{Bongabdo, CalendarStandard};

let date = Bongabdo::from_ymd(2024, 3, 14, CalendarStandard::BangladeshLegacy)?;
assert_eq!((date.year, date.month, date.day), (1430, 11, 31));
# Ok::<(), bongabdo::BongabdoError>(())

Use CalendarStandard::WestBengalTraditional when you want the traditional solar calendar used in West Bengal and commonly in India-facing Bengali date contexts.

use bongabdo::{Bongabdo, CalendarStandard};

let date = Bongabdo::from_ymd(2026, 4, 15, CalendarStandard::WestBengalTraditional)?;
assert_eq!((date.year, date.month, date.day), (1433, 1, 1));
# Ok::<(), bongabdo::BongabdoError>(())

Parsing `BD`, `WB`, `IN`, And Other Aliases

CalendarStandard implements FromStr, so you can accept string input in your own API and map it into a strongly typed standard.

use bongabdo::CalendarStandard;

let bd: CalendarStandard = "BD".parse()?;
let legacy: CalendarStandard = "BD_LEGACY".parse()?;
let wb: CalendarStandard = "WB".parse()?;
let india: CalendarStandard = "IN".parse()?;

assert_eq!(bd, CalendarStandard::Bangladesh);
assert_eq!(legacy, CalendarStandard::BangladeshLegacy);
assert_eq!(wb, CalendarStandard::WestBengalTraditional);
assert_eq!(india, CalendarStandard::WestBengalTraditional);
# Ok::<(), Box<dyn std::error::Error>>(())

These aliases are supported:

BD, BANGLADESH, BANGLADESH_MODERN, BD_MODERN
BD_LEGACY, BANGLADESH_LEGACY, BANGLADESH_1987
WB, WEST_BENGAL, WESTBENGAL, WEST_BENGAL_TRADITIONAL
IN, INDIA, INDIAN_BENGALI, INDIA_BENGALI

Converting Explicit Gregorian Dates

Bongabdo::from_ymd(...) is the most deterministic API in the crate.

use bongabdo::{Bongabdo, CalendarStandard};

let date = Bongabdo::from_ymd(2026, 12, 16, CalendarStandard::Bangladesh)?;
assert_eq!(date.year, 1433);
assert_eq!(date.month, 9);
assert_eq!(date.day, 1);
# Ok::<(), bongabdo::BongabdoError>(())

This is the best choice when:

you already have a Gregorian date
you want reproducible behavior in tests
you do not want any dependence on the current machine clock

Converting The Current Date

Bongabdo::today(...) computes the Bengali date for the current local civil day of the chosen standard.

Bangladesh standards use the current Dhaka civil offset, UTC+06:00
West Bengal uses the current Kolkata civil offset, UTC+05:30

This is a fixed-offset model. The crate does not currently use IANA timezone database history for timestamp conversion.

use bongabdo::{Bongabdo, CalendarStandard};

let today_bd = Bongabdo::today(CalendarStandard::Bangladesh)?;
let today_wb = Bongabdo::today(CalendarStandard::WestBengalTraditional)?;

assert_eq!(today_bd.standard, CalendarStandard::Bangladesh);
assert_eq!(today_wb.standard, CalendarStandard::WestBengalTraditional);
# Ok::<(), bongabdo::BongabdoError>(())

Converting From `SystemTime`

If you already have a fixed timestamp, use Bongabdo::from_system_time(...).

use bongabdo::{Bongabdo, CalendarStandard};
use std::time::{Duration, UNIX_EPOCH};

let timestamp = UNIX_EPOCH + Duration::from_secs(1_776_105_900);
let wb = Bongabdo::from_system_time(timestamp, CalendarStandard::WestBengalTraditional)?;

assert_eq!(wb, Bongabdo::from_ymd(2026, 4, 14, CalendarStandard::WestBengalTraditional)?);
# Ok::<(), bongabdo::BongabdoError>(())

This is useful for:

backend services with timestamp-based data
tests around local date rollover
replaying historical conversions from stored timestamps

This is not a timezone-history-accurate API. It applies the current civil offset for the chosen standard rather than full timezone database rules.

Accessing Date Fields

Each Bongabdo value gives you direct access to the computed components.

use bongabdo::{Bongabdo, CalendarStandard};

let date = Bongabdo::from_ymd(2026, 4, 14, CalendarStandard::Bangladesh)?;

assert_eq!(date.year, 1433);
assert_eq!(date.month, 1);
assert_eq!(date.day, 1);
assert_eq!(date.weekday, 2);
assert_eq!(date.standard, CalendarStandard::Bangladesh);
# Ok::<(), bongabdo::BongabdoError>(())

Weekday numbering is:

0 = Sunday
1 = Monday
2 = Tuesday
3 = Wednesday
4 = Thursday
5 = Friday
6 = Saturday

Month And Weekday Names

You can access both Bengali-script and Roman-script names.

use bongabdo::{Bongabdo, CalendarStandard};

let date = Bongabdo::from_ymd(2026, 4, 14, CalendarStandard::Bangladesh)?;

assert_eq!(date.month_name(), "বৈশাখ");
assert_eq!(date.month_name_roman(), "Boishakh");
assert_eq!(date.weekday_name(), "মঙ্গলবার");
assert_eq!(date.weekday_name_roman(), "Mangalbar");
# Ok::<(), bongabdo::BongabdoError>(())

Formatting

Bengali Output

Use to_bengali_string() when you want Bengali digits and Bengali script.

use bongabdo::{Bongabdo, CalendarStandard};

let date = Bongabdo::from_ymd(2026, 4, 14, CalendarStandard::Bangladesh)?;
assert_eq!(date.to_bengali_string(), "মঙ্গলবার ১ বৈশাখ, ১৪৩৩");
# Ok::<(), bongabdo::BongabdoError>(())

Roman Output

Use to_roman_string() when you want Roman-script month and weekday names.

use bongabdo::{Bongabdo, CalendarStandard};

let date = Bongabdo::from_ymd(2026, 4, 15, CalendarStandard::WestBengalTraditional)?;
assert_eq!(date.to_roman_string(), "Budhbar 1 Boishakh, 1433");
# Ok::<(), bongabdo::BongabdoError>(())

`Display` And Standard `to_string()`

Bongabdo implements Display, so the standard Rust ToString::to_string() output is the Roman-script form.

use bongabdo::{Bongabdo, CalendarStandard};

let date = Bongabdo::from_ymd(2026, 4, 14, CalendarStandard::Bangladesh)?;

assert_eq!(date.to_string(), "Mangalbar 1 Boishakh, 1433");
assert_eq!(format!("{date}"), "Mangalbar 1 Boishakh, 1433");
# Ok::<(), bongabdo::BongabdoError>(())

Error Handling

Invalid Gregorian input returns BongabdoError::InvalidGregorianDate.

use bongabdo::{Bongabdo, BongabdoError, CalendarStandard};

let result = Bongabdo::from_ymd(2026, 2, 29, CalendarStandard::Bangladesh);

assert_eq!(
    result,
    Err(BongabdoError::InvalidGregorianDate {
        year: 2026,
        month: 2,
        day: 29,
    })
);

Parsing an unsupported alias returns an error from FromStr.

use bongabdo::CalendarStandard;

assert!("XX".parse::<CalendarStandard>().is_err());

Backwards Compatibility

Bongabdo::now() is still available for compatibility with older versions of the crate, but it is intentionally deprecated because it is ambiguous. It currently maps to CalendarStandard::BangladeshLegacy.

For new code, prefer:

Bongabdo::today(CalendarStandard::Bangladesh)
Bongabdo::today(CalendarStandard::BangladeshLegacy)
Bongabdo::today(CalendarStandard::WestBengalTraditional)

Notes

WB and IN are aliases for the same traditional solar standard.
West Bengal support is implemented as a separate traditional solar calculation, not as a one-day offset from Bangladesh.
The West Bengal path is modeled from sidereal solar ingress. For exact archival or panjika-grade historical work, verify important boundaries against an authoritative published panjika.
from_ymd(...) is the best API for deterministic conversions.
today(...) and from_system_time(...) use fixed current civil offsets, not historical timezone database rules.
today(...) and from_system_time(...) are appropriate when present-day "current day" or timestamp-based behavior matters.

Features

Convert explicit Gregorian dates with Bongabdo::from_ymd(...)
Convert the current local date with Bongabdo::today(...)
Convert timestamps with Bongabdo::from_system_time(...)
Parse BD, WB, IN, and related aliases into CalendarStandard
Support Bangladesh modern, Bangladesh legacy, and West Bengal traditional standards
Format dates in Bengali and Roman script
Access individual date components, month names, and weekday names

License

This project is licensed under the MIT License. See LICENSE for details.

bongabdo 0.1.0