A year of benefits coverage under an individual health insurance plan. The benefit year for plans bought inside or outside the Marketplace begins January 1 of each year and ends December 31 of the same year. Your coverage ends December 31 even if your coverage started after January 1. Any changes to benefits or rates to a health insurance plan are made at the beginning of the calendar year.

References

https://www.healthcare.gov/glossary/benefit-year/