/**
 * Calculate the tier start date from the tier expiration date.
 * The tier period is 1 year, so start date is expiration minus 1 year.
 */
export function getTierStartDate(
  tierExpirationDate: string | null | undefined
): string | null {
  if (!tierExpirationDate) return null

  const date = new Date(tierExpirationDate)
  date.setFullYear(date.getFullYear() - 1)

  return date.toISOString().split("T")[0]
}