Arithmetic & ranges

Adding and subtracting days and months, spans between dates, month/year boundaries, and age.

Days

function addBsDays(date: BSDate, days: number): BSDate

The date days calendar days after date (or before, if days is negative). Throws InvalidBSDateError if date is invalid, RangeError if days isn't an integer, and DateOutOfRangeError if the result falls outside BS 1979–2100.

function subtractBsDays(date: BSDate, days: number): BSDate

The date days calendar days before date. The same as addBsDays(date, -days).

function daysBetweenBs(from: BSDate, to: BSDate): number

The number of calendar days from from to to: positive if to is later, negative if earlier, zero if equal. The argument order matches go-bs's DaysBetween(a, b): the result is to − from.

function getBsDayOfWeek(date: BSDate): Weekday

The day of the week date falls on, numbered like Date#getDay() (0 is Sunday).

import { addBsDays, daysBetweenBs, getBsDayOfWeek, subtractBsDays } from "bikram-sambat-ts";
 
const d = { year: 2083, month: 6, day: 6 };
 
addBsDays(d, 10); // { year: 2083, month: 6, day: 16 }
subtractBsDays(d, 10); // { year: 2083, month: 5, day: 27 }
addBsDays(d, 1); // { year: 2083, month: 6, day: 7 }
 
getBsDayOfWeek(d); // 2 (Tuesday)
 
daysBetweenBs(d, { year: 2083, month: 6, day: 16 }); // 10

Months

function nextBsMonth(date: BSDate): BSDate

The date one BS month after date, on the same day of the month — clamped to the target month's last day if it's shorter, rather than rolling over into the month after.

function previousBsMonth(date: BSDate): BSDate

The date one BS month before date, with the same clamping.

⚠

The clamping is deliberate, and matches go-bs's NextMonth/PreviousMonth: day 32 in a 32-day month becomes day 31 in a 31-day target month, not day 1 of the month after. Past either end of the range, both throw InvalidBSDateError with field: "year", like go-bs's ErrInvalidYear (whereas addBsDays throws DateOutOfRangeError, like go-bs's AddDays).

import { nextBsMonth, previousBsMonth } from "bikram-sambat-ts";
 
nextBsMonth({ year: 2083, month: 6, day: 6 }); // { year: 2083, month: 7, day: 6 }
previousBsMonth({ year: 2083, month: 6, day: 6 }); // { year: 2083, month: 5, day: 6 }
nextBsMonth({ year: 2083, month: 12, day: 15 }); // { year: 2084, month: 1, day: 15 }
 
// Ashadh (month 3) 2083 has 32 days; Shrawan (month 4) has only 31.
nextBsMonth({ year: 2083, month: 3, day: 32 }); // { year: 2083, month: 4, day: 31 }

Month and year boundaries

function startOfBsMonth(date: BSDate): BSDate

Day 1 of date's month.

function endOfBsMonth(date: BSDate): BSDate

The last day (29–32) of date's month.

function startOfBsYear(date: BSDate): BSDate

Baisakh 1 of date's year.

function endOfBsYear(date: BSDate): BSDate

The last day of Chaitra in date's year.

function getBsDayOfYear(date: BSDate): number

date's day of the year: 1 for Baisakh 1, up to 365 or 366 for the last day of Chaitra.

import { endOfBsMonth, endOfBsYear, getBsDayOfYear, startOfBsMonth, startOfBsYear } from "bikram-sambat-ts";
 
const d = { year: 2083, month: 6, day: 6 };
 
startOfBsMonth(d); // { year: 2083, month: 6, day: 1 }
endOfBsMonth(d); // { year: 2083, month: 6, day: 31 }
startOfBsYear(d); // { year: 2083, month: 1, day: 1 }
endOfBsYear(d); // { year: 2083, month: 12, day: 30 } (Chaitra 2083 has 30 days)
getBsDayOfYear(d); // 162

These validate the whole date, including the day. go-bs's versions only check the year and month.

Age

function getBsAge(birth: BSDate, today?: BSDate): BSAge

The calendar age from birth to today as { years, months, days }, counted in BS months — real calendar math with month/day borrowing, the same algorithm as go-bs's Age, not a day count divided by 365. today defaults to todayBs(). Throws InvalidDateOrderError if birth is after today.

import { getBsAge } from "bikram-sambat-ts";
 
const birth = { year: 2060, month: 6, day: 15 };
 
getBsAge(birth, { year: 2083, month: 6, day: 6 }); // { years: 22, months: 11, days: 22 }
getBsAge(birth); // age as of today in Nepal