The timeline is that way because they're assuming that Black Lightning returning happened closer to the series premiere. Given there are changes like the 2015 Rip date as well as the Particle Accelerator accident being in 2014 (I'm just assuming that given these events happened so close to a new year that their placements look like they're in these years even though they aren't), I don't think the timeline needs to be followed. Especially since it was more in the background and not an actual part of the plot. Any contradiction can be explained away as a post-Crisis change. I think the first three episodes of Supergirl being around Early October with the next two being in Mid-October work pretty well. No matter what, I wouldn't leave them as being over a few days.2018: Batwoman debuts. Black Lightning returns. Thunder debuts. President Marsdin is exposed as an alien
In that order.
I'm ignoring the 2015 date for Rip Hunter forming the Legends because there is contradictory evidence of that not being the case; which could be chalked up to slight changes in a post-Crisis timeline.