intervalOverlappingDaysDateTime

Signature

intervalOverlappingDaysDateTime(aStart: string, aEnd: string, bStart: string, bEnd: string): number

import { intervalOverlappingDaysDateTime } from "@northguild/gmt/plain/interval";

Return how many distinct calendar dates two datetime intervals share.

• Counts the number of local dates touched by the closed intersection [max(aStart, bStart), min(aEnd, bEnd)] — inclusive of both endpoints.
• Adjacent intervals (e.g. aEnd === bStart) share one date and count as 1.
• Returns 0 when the intervals do not overlap at all (a well-defined answer, not invalid input).
• Returns null if either interval is invalid (start > end).
• Returns null on invalid input (wrong type, malformed strings).
• Diverges from date-fns’s getOverlappingDaysInIntervals, which rounds up elapsed 24-hour periods instead of counting calendar dates (its own doc example — Jan 10-20 vs Jan 17-21 — returns 3 there, 4 here). To reproduce date-fns’s number, compose intervalIntersectionDateTime with intervalCountDateTime: const span = intervalIntersectionDateTime(aStart, aEnd, bStart, bEnd); span ? intervalCountDateTime(span.start, span.end, "day") : 0;

Parameters

Parameter	Type	Description
aStart	string	ISO 8601 datetime string for the first interval start
aEnd	string	ISO 8601 datetime string for the first interval end
bStart	string	ISO 8601 datetime string for the second interval start
bEnd	string	ISO 8601 datetime string for the second interval end

Returns

number of shared calendar dates, 0 when disjoint, or null on invalid input

Examples

intervalOverlappingDaysDateTime("2024-01-01T23:59:00", "2024-01-02T00:01:00", "2024-01-01T23:59:00", "2024-01-02T00:01:00") // 2
intervalOverlappingDaysDateTime("2024-01-01T00:00:00", "2024-01-02T00:00:00", "2024-01-02T00:00:00", "2024-01-03T00:00:00") // 1 (adjacent)
intervalOverlappingDaysDateTime("2024-01-01T00:00:00", "2024-01-02T00:00:00", "2024-01-02T00:00:00.001", "2024-01-03T00:00:00") // 0 (disjoint)
intervalOverlappingDaysDateTime("invalid", "2024-06-30T23:59:59", "2024-04-01T00:00:00", "2024-12-31T23:59:59") // null

Source

plain/interval/intervalOverlappingDaysDateTime.ts