Most countries align the fiscal year with the calendar year. In other words, the fiscal year begins on January 1 and ends on December 31.
Other countries have decided to desynchronize calendars by default. As a general rule, the fiscal year starts on the first day of the month. Some exceptions exist, such as Iran (21 March), Ethiopia (7 July), and Nepal (16 July), etc.
The fiscal year can be determined either before connecting your billing software or at any time in the parameters related to your organization.