The beginning date for an accounting reporting period is the day after your inventory is filed, or the day after the the ending date of the last accounting reporting period for which you actually filed.
If you have never filed an inventory or accounting report, then the reporting period starts with the day after the date of your appointment as a guardian and/or conservator, even if you were appointed a number of years ago.
The ending date for a reporting period is the anniversary date of your appointment as guardian and/or conservator, unless you have a court order changing the date.
The reason to change a reporting period might be if you are required to submit a similar report somewhere else, say to Social Security or the IRS, and because the report dates are different you are having to prepare two separate reports at two different times each year, with similar information in each report. If you change you accounting reporting period with the court to coincide with your other reporting period, then you can prepare both reports together at the same time each year.