create measure last year to date in LookML

[tancco-16560046]

I’m trying to create a last year measure, I have this:
  dimension_group: date_orders {
    type: time
    timeframes: [
      raw,
      date,
      week,
      month,
      quarter,
      year
    ]
    convert_tz: no
    datatype: date
    sql: ${TABLE}.Date_Orders ;;
  }

measure: total_units {
    type: sum
    sql: ${units} ;;
  }

and I need:

measure: total_units_ly {
    but I don’t know how to define it
  }