That’s the day when 4.5M people served by Boston’s TV stations who pre-register will be able to stream local broadcast programming from Aereo. Others in Massachusetts, New Hampshire and Vermont will be able to subscribe at the end of May. This is the first time Aereo will offer its service outside the New York City area, and is part of the company’s announced plan to expand to 22 markets in 2013. But it also could intensify the Barry Diller-backed company’s fight with broadcasters: It will stream programming from 28 over-the-air channels in Boston without their permission. (Aereo also has an agreement with Bloomberg Television to offer its pay TV channel.) Broadcasters in New York have already taken Aereo to court, alleging that it violates their copyrights. The company counters that it simply rents antennas, enabling subscribers to watch programming that’s already available to them for free. It adds that consumers also have the right to stream their content, much like they would if they bought a Slingbox. Today’s announcement follows an appeals court ruling that enables Aereo to remain in business while the broadcasters’ case proceeds. Execs at News Corp, CBS, and Univision have said that they might convert their over-the-air channels to pay TV services if they don’t prevail in court. Aereo has offices in Boston’s Innovation District and says it employees more than 60 people there, mostly engineers and developers.