Final answer:
Custom fiscal years are periods that follow the Gregorian calendar but can start on the first day of any month.
Step-by-step explanation:
The periods that follow the Gregorian calendar but can start on the first day of any month are called custom fiscal years. Custom fiscal years are used by some organizations and businesses to align their financial reporting periods with their operational needs. By choosing a custom fiscal year, these entities can start their fiscal year on a specific date that makes sense for them, rather than being bound to the standardized calendar year.