This is the earliest month for which you want to record transactions. For example, if you want to enter transactions dated 1 September or later, your conversion month is September. You cannot enter transactions dated before the conversion month.
You can choose to have 12 or 13 accounting periods. A 13th period is a period for recording entries which you do not want included in the accounting figures for a particular month. For example, you can use it to record year-end adjustments that are attributed to all months. If in doubt, choose 13 periods.