payroll.pro-rata-salary Unreviewed

export function proRataSalary(periodSalary: Money, fteBasisPoints: number, periodStart: string, periodEnd: string, workedFrom: string, workedTo: string, method: ProRataMethod, holidays: readonly string[]): ProRata

export type ProRataMethod = "calendar-days" | "working-days";

/** The amount and the fraction it came from. */
export interface ProRata {
  readonly pay: Money;
  /** days in the employed part of the period, by the method */
  readonly daysCounted: number;
  /** days in the whole period, by the same method */
  readonly daysInPeriod: number;
}

import { proRataSalary } from "#fune/payroll.pro-rata-salary@^1";

import { addDays, daysBetween } from "./dates_add_days.ts";  ← from dates.add-days ^1.0.0 · built alongside by fune
import { businessDaysBetween } from "./dates_business_days_between.ts";  ← from dates.business-days-between ^1.0.0 · built alongside by fune
import { roundDiv } from "./math_round_div.ts";  ← from math.round-div ^1.0.0 · built alongside by fune
import { type Money, money } from "./money_amount.ts";  ← from money.amount ^1.0.0 · built alongside by fune
import { type ProRata, type ProRataMethod } from "./payroll_pro_rata_salary_types.ts";

function countDays(from: string, to: string, method: ProRataMethod, holidays: readonly string[]): number {
  if (method === "calendar-days") return daysBetween(from, to) + 1;
  if (method === "working-days") return businessDaysBetween(from, addDays(to, 1), holidays);
  throw new RangeError(`unknown pro-rata method "${method}"`);
}

/** Pro-rate a period's salary by time employed and FTE, rounding once. */
export function proRataSalary(
  periodSalary: Money,
  fteBasisPoints: number,
  periodStart: string,
  periodEnd: string,
  workedFrom: string,
  workedTo: string,
  method: ProRataMethod,
  holidays: readonly string[],
): ProRata {
  if (!Number.isInteger(fteBasisPoints) || fteBasisPoints < 1 || fteBasisPoints > 10000) {
    throw new RangeError(`fteBasisPoints must be a whole number from 1 to 10000, received ${fteBasisPoints}`);
  }
  if (daysBetween(periodStart, periodEnd) < 0) throw new RangeError("periodEnd is before periodStart");
  if (daysBetween(workedFrom, workedTo) < 0) throw new RangeError("workedTo is before workedFrom");
  if (workedFrom < periodStart || workedTo > periodEnd) {
    throw new RangeError("the worked dates must fall inside the pay period");
  }
  const daysInPeriod = countDays(periodStart, periodEnd, method, holidays);
  if (daysInPeriod === 0) throw new RangeError("the pay period has no working days to pro-rate over");
  const daysCounted = countDays(workedFrom, workedTo, method, holidays);
  const minor = roundDiv(periodSalary.minor * fteBasisPoints * daysCounted, 10000 * daysInPeriod, "half-up");
  return { pay: money(minor, periodSalary.currency), daysCounted, daysInPeriod };
}

Install

fune build

With that line in your source, in a TypeScript project (language typescript in fune.project), fune build resolves it and its 4 dependencies, pins them in fune.lock, downloads only the TypeScript package of each, and builds the code above into your project’s .fune/build, one readable file per capability with a header linking back here. Or pin a range in fune.project and build in one step:

fune add payroll.pro-rata-salary

The manifest, vectors and README with only the TypeScript implementation. Install it without the registry with fune add ./payroll.pro-rata-salary-1.0.1-typescript.fune, or fetch it from a terminal with fune pull payroll.pro-rata-salary@1.0.1:typescript.

The whole function, every language, is one file too: payroll.pro-rata-salary-1.0.1.fune, 15,940 bytes, sha256 f8bb34881684fca05492f85329cff6f12cb9fef7c6277d1903e38ba24f5bfdf0. It installs into a project of any language.

// fune: before payroll.pro-rata-salary

// fune: after payroll.pro-rata-salary

// fune: replace dates.add-days in payroll.pro-rata-salary
// fune: replace dates.business-days-between in payroll.pro-rata-salary
// fune: replace math.round-div in payroll.pro-rata-salary
// fune: replace money.amount in payroll.pro-rata-salary

// fune: step payroll.pro-rata-salary after <n|label>

More from the author

computed exactly and rounded half-up to the penny once, at the end (math.round-div), so the fraction of a penny is never rounded twice.

Two methods, because employers use both and they give different answers:

- **calendar-days** counts every day: 14 to 30 September is 17 of 30 days. - **working-days** counts Monday to Friday, less the `holidays` supplied (dates.business-days-between): 14 to 30 September 2026 is 13 of 22 days.

Which one applies is a contractual choice, not a legal one. A third common method, 1/260th or 1/365th of the annual salary per day, is the calendar or working-days method applied to a year-long period: pass the annual salary with the tax year as the period.

Both date ranges are inclusive. The employed range must sit inside the pay period. A working-days period with no working days in it is an error, since the fraction would be 0/0.

## Before you rely on this

**Not professional advice.** This capability calculates payroll figures from published rules. It is a software component for developers, not tax or legal advice. Rules change and every rate here has an effective date. Check that the dates cover your case. Verify results against the official sources listed above, and have a payroll specialist review how you use it, before anyone relies on the output. Provided "as is" under its licence, without warranty.

**Unreviewed.** This capability's implementations agree in every language and pass its published test vectors, which were worked out from the official sources cited. But no qualified payroll specialist has yet checked those vectors, or confirmed that the capability covers the cases it claims. Treat it as a draft. Do not use it for real people, money or decisions without your own expert review. Once a qualified reviewer signs off, this notice is replaced with their name, qualification and the date. Each new version needs fresh sign-off.

1.0.1 marks it unreviewed. The code and the tests are unchanged.

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