如何在Haskell中获取上个月的开始时间？

import Data.Time.Calendar
import Data.Time.LocalTime

getStartOfLastMonth :: IO LocalTime
getStartOfLastMonth = do
  now <- getZonedTime
  let (year, month, _) = toGregorian $ localDay $ zonedTimeToLocalTime now
      lastMonth = fromGregorian (year - if month == 1 then 1 else 0) (if month == 1 then 12 else month - 1) 1
  return $ LocalTime lastMonth midnight

main :: IO ()
main = do
  startOfLastMonth <- getStartOfLastMonth
  putStrLn $ "Start of last month: " ++ show startOfLastMonth