Files incoming messages into subfolders of Inbox by YYYY-MM

fileinto_subfolder_by_date.sieve

require ["date", "fileinto", "variables"];

if date :matches "date" "year" "*" {
  set "year" "${1}";
}

if date :matches "date" "month" "*" {
  if string :matches "${1}" "?" {
    set "month" "0${1}";
  } else {
    set "month" "${1}";
  }
}

fileinto "INBOX.${year}-${month}";
stop;