The plan start month column is optional for the 2015 reporting year, but by entering any code in this slot, there is a built in verification that will error if a plan start month of "00" is entered with an invalid code. The only time 00 should be entered is if there is no offer of coverage and the employee has code 1H selected.  

Otherwise, when you, the employer, have offered an employee coverage and they don't take advantage of that coverage, you can either leave that field blank for 2015 or enter the calendar month when the plan year of the health plan begins when the employee is offered coverage. "Start" in this case should refer to when the plan actually takes affect as opposed to when they can start selecting their coverage - typically Jan-Dec.

Plan Start Month:
This box is optional for the 2015 Form 1095-C and the employer may leave it blank; it is anticipated that this box will be mandatory for the 2016 Form 1095-C. To complete the box, 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”.

For additional resources, please reference: https://www.irs.gov/pub/irs-prior/i109495c--2015.pdf