Untitled

lets(
  dateText, 31051965.format(),
  forloop, "x".repeat(dateText.length()).split("x"),

  result, forloop.map(
    dateText.substring(
      index, 
      index+1
    ).toNumber()
  ).sum(),
  ten, round(result/10),

  if(
    result < 10,result,
    ten + result - ten*10
  )
)