Dates and day counts

Module: Shoals.Date.

This module computes year fractions under the standard day-count conventions, detects weekends, rolls a date to a business day under the following / modified-following / preceding rules, and generates a schedule of dates stepped by a number of months. The date type itself comes from Std.Time, constructed with date(year, month, day).

Day-count conventions

type DayCount =
  | Act360
  | Act365
  | ThirtyThreeSixty
  | ActAct

def year_fraction(start: Date, end: Date, convention: DayCount) -> f32

year_fraction returns the year fraction between two dates under the chosen convention. Act360 divides actual days by 360, Act365 divides by 365, ThirtyThreeSixty is the 30/360 bond-basis count, and ActAct divides actual days by 365.25. From tests/date.ch:

start = date(cast(2025, int64), cast(1, int64), cast(1, int64))
end = date(cast(2026, int64), cast(1, int64), cast(1, int64))
yf = year_fraction(start, end, Act365)  // yf == 1.0

half = year_fraction(
  date(cast(2025, int64), cast(1, int64), cast(15, int64)),
  date(cast(2025, int64), cast(7, int64), cast(15, int64)),
  ThirtyThreeSixty
)  // half == 0.5

Weekends and business-day rolling

def is_weekend(d: Date) -> bool
def date_roll_following(d: Date, weekend_only: bool) -> Date
def date_roll_modified_following(d: Date, weekend_only: bool) -> Date
def date_roll_preceding(d: Date, weekend_only: bool) -> Date
def add_business_days(d: Date, n: int64, weekend_only: bool) -> Date

is_weekend reports whether a date falls on Saturday or Sunday. date_roll_following advances a weekend date forward to the next weekday, date_roll_preceding retreats it to the previous weekday, and date_roll_modified_following rolls forward unless that crosses into the next month, in which case it rolls back. add_business_days steps forward n days, skipping weekends. From tests/date.ch, a Saturday rolls to the following Monday:

sat = date(cast(2025, int64), cast(1, int64), cast(4, int64))
rolled = date_roll_following(sat, true)
// rolled is Monday 2025-01-06

The weekend_only flag indicates that only weekends are skipped; this module does not consult a holiday list. To roll against holidays as well, combine it with the holiday calendars.

Schedule generation

def schedule_from_tenor(start: Date, end: Date, step_months: int64) -> List[Date]

schedule_from_tenor returns a list of dates from start, stepped by step_months months, up to and including the last stop at or before end. The first entry is the start date and the schedule is strictly increasing. From tests/date.ch, a quarterly schedule across 2025 has five stops:

dates = schedule_from_tenor(
  date(cast(2025, int64), cast(1, int64), cast(1, int64)),
  date(cast(2025, int64), cast(12, int64), cast(31, int64)),
  cast(3, int64)
)
// len(dates) == 5

Months are treated as a fixed thirty-day step in this schedule generator.