Timestamp Converter — TypeScript source

// Pure surface of the timestamp tool (slug: timestamp). The island renders
// rows via locale-aware Date builtins; the genuinely portable logic is the
// human relative-time breakdown (thresholds, rounding, direction). `now` is
// injected so tests are deterministic; the thresholds mirror the island's
// original behavior exactly.
export type RelativeUnit = 'sec' | 'min' | 'hr' | 'day' | 'month' | 'yr';
export type RelativeDirection = 'past' | 'future';

export interface RelativeBreakdown {
  /** Magnitude in `unit` (rounded). */
  n: number;
  unit: RelativeUnit;
  direction: RelativeDirection;
}

const MIN = 60_000;
const HR = 3_600_000;
const DAY = 86_400_000;
const MONTH = DAY * 30;
const YEAR = DAY * 365;

/** Break |ms - now| into the coarsest unit under its threshold.
 *  0 → { n: 0, unit: 'sec', direction: 'future' } ("in 0 sec"), matching the
 *  island's original rendering. */
export function relativeBreakdown(ms: number, now: number = Date.now()): RelativeBreakdown {
  const diff = ms - now;
  const abs = Math.abs(diff);
  let n: number;
  let unit: RelativeUnit;
  if (abs < MIN) {
    n = Math.round(abs / 1000);
    unit = 'sec';
  } else if (abs < HR) {
    n = Math.round(abs / MIN);
    unit = 'min';
  } else if (abs < DAY) {
    n = Math.round(abs / HR);
    unit = 'hr';
  } else if (abs < MONTH) {
    n = Math.round(abs / DAY);
    unit = 'day';
  } else if (abs < YEAR) {
    n = Math.round(abs / MONTH);
    unit = 'month';
  } else {
    n = Math.round(abs / YEAR);
    unit = 'yr';
  }
  return { n, unit, direction: diff < 0 ? 'past' : 'future' };
}