As they've done with key episodes in the past, The CW has announced that the upcoming crossover episodes of The Flash and Arrow, branded as "Heroes Join Forces," will be expanded.
The Flash will air from 8 p.m. ET/PT until 9:01:30 p.m. on Tuesday, December 1. Arrow will air from 8 p.m. ET/PT until 9:02:30 p.m. on Wednesday, December 2.
No word on why the episodes are expanded. In the past, such expansions have often (but not always) been to include a preview of upcoming developments in the shows -- and the two-and-a-half-minute Legends of Tomorrow preview released yesterday would certainly fit into the space at the end of Arrow. Sometimes, though, they're just expanded because the episode needs a little more time to breathe.