The Plan Start Month is Optional for tax years 2015 and 2016.

This enhancement was included in Sage 100 2015 Product Update 4 and Sage 100 2016 Product Update 1 and higher.

The plan start month will not pull into ACA preparer as of 2016 year filings. It is planned on being mandatory per irs link below in 2017.

If you desire to populate the 1095C, you can manually enter the 2 digit month in Aatrix ACA Preparer

The plan start month is held in the Payroll, ACA, ACA Employee Maintenance

1. Select applicable Employee No and Tax Year
2. In the Plan Start Month field, enter the two-digit number (01 through 12) indicating the calendar month during which the plan year begins of the health plan in which the employee is offered coverage (or would be offered coverage if the employee were eligible to participate in the plan). If more than one plan year could apply (for instance, if the employer changes the plan year during the year), enter the earliest applicable month. If there is no health plan under which coverage is offered to the employee, enter "00" (which is the default)
3. Click Accept

For more information on the Plan Start Date see IRS instructions, https://www.irs.gov/pub/irs-pdf/i109495c.pdf