What is plan start month for Form 1095-C?

Plan start month is the month the employer's plan year (that was offered to the employee) starts, not the month that the employee enrolls in the coverage.

This should be input using the values 01-12 to represent the months of the year, and 00 to represent that no offer of coverage was made.

For example, if the plan offered starts in January, then this would be 01 in the plan start month box.