addHelper (fnName, fn) {
    // check existed helper fn
    /* istanbul ignore if */
    if (fnName in locale.helpers) {
      warn(`${fnName} has already been registered on helpers function, please change another name`)
      return
    }
    locale.helpers[fnName] = fn
  }
Safari don't support formatRules like
'yyyy-MM-dd hh:mm:ss', so transfer it to 'yyyy/MM/dd hh:mm:ss'