Holiday-aware business-day calculator for 200+ countries. Three modes: pass start + addDays (signed integer) to shift a date by N business days; pass start + end to count business days between two dates (exclusive of start, inclusive of end); pass start alone to check one date (is it a business day, which holiday, next/previous business day). Query: country (ISO 3166-1 alpha-2), start (YYYY-MM-DD), optional region (subdivision code), addDays XOR end, optional weekend (comma-separated days, default sat,sun — e.g. fri,sat for the Gulf), optional types (holiday types treated as closures, default public,bank). Skipped holidays are itemized in the response. Use for payment terms, SLA deadlines, and delivery-date math.