In Notice 2015-17 Q&A-1, “(2) for January 1 through June 30, 2015 for employers that are not ALEs for 2015. After June 30, 2015, such employers MAY be liable for the Code § 4980D excise tax.”
What does it mean by MAY be liable? Is the date final?
This means that if you are reimbursing an employee for individual Marketplace coverage you don't have to apply for the exemption for this until after June 30, 2015. At that point an employer DOES have to apply for an exemption for the excise tax. Employers who don't have to comply with the mandate are exempt from the excise tax (believe they would still need to file for the exemption post June 30). You can read about other details here.
The tax is due to the fact that as an employer who has to offer coverage you can't have your employees getting Marketplace coverage and therefore potentially getting tax credits. If you do this post June 30, and you are a large employer, you may get the $36,500 per employee, per year fee. MAY because you can file for an exemption and correct yourself and avoid it. The fee isn't to punish or collect money from those confused, it's to have a big deterrent for those willingly breaking this rule.
The best defense is offering a group health plan and pairing that with an employer payment plan.