/** ISO string for a Date, keeping nulls. */
export const iso = <T extends Date | null>(date: T): T extends Date ? string : string | null => (date ? date.toISOString() : null) as never

/** Calendar date in the server's time zone (set TZ to the plant's zone), e.g. `2026-10-06`. */
export function localDate(date = new Date()): string {
  const pad = (value: number) => String(value).padStart(2, '0')
  return `${date.getFullYear()}-${pad(date.getMonth() + 1)}-${pad(date.getDate())}`
}

export function addDays(isoDate: string, days: number): string {
  const date = new Date(`${isoDate}T12:00:00`)
  date.setDate(date.getDate() + days)
  return localDate(date)
}