/**
 * Count how many `unit` boundaries a date-time interval crosses.
 *
 * - Counts calendar boundaries touched by the half-open interval `[start, end)` — distinct
 *   from `diffDateTime`, which measures exact elapsed duration. An interval from 23:59 to
 *   00:01 is two minutes long but touches 2 day boundaries.
 * - The end boundary is excluded: `"2024-01-01T00:00:00"` to `"2024-01-03T00:00:00"` counts 2 days.
 * - A zero-length interval counts 1 when it sits mid-unit and 0 when it sits exactly on a
 *   unit boundary.
 * - Weeks start on Monday (ISO 8601).
 * - Accepts singular or plural units (`"day"` and `"days"` behave identically).
 * - Returns `null` on invalid input (unparseable start/end, `start > end`, unsupported unit).
 *
 * @param start ISO PlainDateTime string for the interval start
 * @param end ISO PlainDateTime string for the interval end
 * @param unit unit string — any `DateTimeUnit`
 * @returns number of unit boundaries touched, or null on invalid input
 *
 * @example intervalCountDateTime("2024-01-01T23:59:00", "2024-01-02T00:01:00", "day") // 2
 * @example intervalCountDateTime("2024-01-01T00:00:00", "2024-01-03T00:00:00", "day") // 2
 * @example intervalCountDateTime("2024-01-01T10:30:00", "2024-01-01T12:00:00", "hour") // 2
 * @example intervalCountDateTime("2024-01-01T05:00:00", "2024-01-01T05:00:00", "day") // 1 (zero-length, mid-day)
 * @example intervalCountDateTime("2024-01-01T00:00:00", "2024-01-01T00:00:00", "day") // 0 (zero-length, on the boundary)
 * @example intervalCountDateTime("invalid", "2024-01-02T00:00:00", "day") // null
 */
export declare function intervalCountDateTime(start: string, end: string, unit: string): number | null;