If your tax year starts on a different day/month for ex. if it begins on the 6th of April and ends on 5th of April, you can enter 6 in the Day box and April in the Month box. Koinly will show you your current tax period under it. A tax year is always 12 months on Koinly and begins at 00:00 on the first day of your tax year and ends at 23:59 on the last day.
The next setting "Timezone for reports" is used to determine the starting time of a day.