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Suites · dates-basics

Dates basics

Calendar arithmetic on ISO dates: add days and months, count days and working days, bank holidays, quarters, recurrence and durations.

21 capabilities (9 core, 12 optional) and 3 gaps, in build order. Put each line in the file that calls it, then fune build.

1. Calendar arithmetic

Move and measure ISO dates exactly, with no date library and no clock reads.

dates.add-days core
Shifts a date by whole days, for due dates and windows.
Shift an ISO date by a whole number of days, forwards or backwards, with exact calendar arithmetic.
import { addDays } from "#fune/dates.add-days@^1";
1.0.0
dates.add-months core
Adds months clamping to month end, so 31 January plus a month is the end of February.
Add calendar months to an ISO date, clamping to the month end: 31 Jan + 1 month is 28 or 29 Feb.
import { addMonths } from "#fune/dates.add-months@^1";
1.0.0
dates.days-between core
Signed calendar days between two dates, for ages of invoices and stays.
Calendar days from one ISO date to another, signed: negative when the second date is earlier.
import { daysBetween } from "#fune/dates.days-between@^1";
1.0.0
dates.month-boundaries core
First and last day of a date's month, for monthly periods and statements.
The first day, last day and length of the calendar month containing a date.
import { monthBoundaries } from "#fune/dates.month-boundaries@^1";
1.0.0
dates.day-of-week core
ISO weekday of a date, for weekend rules and opening hours.
The ISO 8601 day of the week for a date: 1 is Monday through 7 is Sunday.
import { dayOfWeek } from "#fune/dates.day-of-week@^1";
1.0.0
dates.is-leap-year optional
Whether a year is a leap year, for 365/366-day calculations.
Whether a year is a Gregorian leap year: every 4th, except centuries, except every 400th.
import { isLeapYear } from "#fune/dates.is-leap-year@^1";
1.0.0
dates.overlap optional
Days two date ranges share, for prorating and clash checks.
Days two date ranges have in common, with the end of each range stated as inclusive or exclusive.
import { overlapDays } from "#fune/dates.overlap@^1";
1.0.0
dates.age optional
Age in whole years on a date, handling 29 February birthdays.
Age in whole years on a given date, with 29 February birthdays ageing on 1 March in other years.
import { age } from "#fune/dates.age@^1";
1.0.0

2. Periods and reporting dates

Put dates into the periods reports are grouped by.

dates.quarter core
Calendar and fiscal quarter of a date for any year-start month.
Calendar quarter and fiscal quarter of a date, with the fiscal year and quarter date ranges, for any start month.
import { quarter } from "#fune/dates.quarter@^1";
1.0.0
dates.week-number optional
ISO week number and week-year, for weekly reports.
ISO 8601 week number and week-numbering year of a date: weeks start Monday, week 1 holds the first Thursday.
import { weekNumber } from "#fune/dates.week-number@^1";
1.0.0
dates.day-count-fraction optional
Year fraction by ACT/365F, ACT/360, 30/360 or ACT/ACT, for interest periods.
Year fraction between two dates by ACT/365F, ACT/360, 30/360, 30E/360 or ACT/ACT ISDA, as an exact fraction.
import { dayCountFraction } from "#fune/dates.day-count-fraction@^1";
1.0.0

3. Working days and holidays

Skip weekends and public holidays when counting or adding days.

dates.bank-holidays core
UK bank holidays for a region and year, as gov.uk publishes them.
UK bank and public holidays for a region and year, as published by gov.uk, including one-off holidays.
import { bankHolidays } from "#fune/dates.bank-holidays@^1";
1.0.1
dates.add-business-days core
Moves a date by working days, skipping weekends and a holiday list.
Move a date forward or back by working days, skipping weekends and a caller-supplied holiday list.
import { addBusinessDays } from "#fune/dates.add-business-days@^1";
1.0.0
dates.business-days-between core
Working days between two dates, for SLAs and notice periods.
Working days between two dates, excluding weekends and a caller-supplied holiday list.
import { businessDaysBetween } from "#fune/dates.business-days-between@^1";
1.0.0

4. Repeating schedules

Generate the dates of something that repeats.

dates.recurrence optional
The next n dates of a rule: monthly on the 31st, last Friday, every two weeks.
The next n dates of a repeating rule: monthly on a day, last weekday of the month, every k days or weeks, yearly.
import { recurrence } from "#fune/dates.recurrence@^1";
1.0.0

5. Times and durations

Work with times of day, durations and timestamps.

time.duration optional
Parses and adds durations like 1h30m in whole minutes, and formats the total.
Parse durations written as 1h30m, 90m or 01:30, add them up in whole minutes, and format the total.
import { duration } from "#fune/time.duration@^1";
1.0.0
time.minutes-between optional
Minutes between two local date-times, across midnight.
Minutes from one local date and time to another, correct across midnight; wall-clock time, no time zones.
import { minutesBetween } from "#fune/time.minutes-between@^1";
1.0.0
time.round-to-increment optional
Rounds minutes to a billing increment such as 6 or 15.
Round a duration in minutes to a billing increment such as 6 or 15 minutes, up, down or to nearest.
import { roundToIncrement } from "#fune/time.round-to-increment@^1";
1.0.0
time.iso-to-unix optional
RFC 3339 timestamps to Unix seconds.
An RFC 3339 timestamp with a Z or offset, such as 2026-09-26T13:00:00+01:00, to Unix time in whole seconds.
import { isoToUnix } from "#fune/time.iso-to-unix@^1";
1.0.0
time.unix-to-iso optional
Unix seconds to an ISO 8601 UTC timestamp.
Unix time in seconds to an ISO 8601 UTC timestamp such as 2026-09-26T12:00:00Z, for years 0001 to 9999.
import { unixToIso } from "#fune/time.unix-to-iso@^1";
1.0.0
time.countdown optional
Seconds left until a moment and a 4:05 style timer text.
Seconds left until a Unix time, whether it has passed, and a timer text like 4:05, for tokens or retry waits.
import { countdown } from "#fune/time.countdown@^1";
1.0.0

Gaps

What this kind of app usually needs that Functional Weave does not have yet: write these yourself, or use a service.

  • time-zones Time zones and daylight saving: caps works in ISO dates, UTC and wall-clock times; converting with the IANA tz rules is yours.
  • locale-formatting Showing dates in a user's language and format ("3 octobre 2026"): caps returns ISO dates, not localised text.
  • non-uk-holidays Public holidays outside the UK: dates.bank-holidays is UK only; pass another country's list to the working-day functions yourself.