But shouldn't it be the reverse : marked with Sky's dates and alternate list with Now's ?
Considering it is branded as a Sky original, that make more sense.
It's in all the data policies. Now is owned by Sky, so it's still a sky original irregardless.
A show's owner is the entity that commissioned the show. Usually, this is the same entity that airs the show's world premieres: a show commissioned by NBC will premiere on NBC. But occasionally, episodes have their world premiere on an outlet that's completely unrelated to the show's owner.
If a show has a single unambiguous owner throughout its lifetime, the episode airdates should be set to the premiere dates on the owner's TV Network or Web Channel as described above. However, if a show's owner is ambiguous - because it's co-produced by multiple entities - or if the ownership changes throughout its lifetime, the world premiere dates must be used. For example:
Use owner's premiere dates:
The Blacklist (NBC show, occasionally has its world premiere in Canada before shown on NBC)
The Musketeers (BBC show, season 3 premiered in Canada instead of on BBC)
Use world premiere dates:
Deutschland 83 (German show, co-production between Germany and US; premiered in the US)
The Last Kingdom (Season 1 co-production between BBC and BBC America; season 2 co-production between BBC and Netflix)
Black mirror (Season 1 and 2 commissioned by UK's Channel 4; season 3 commissioned by Netflix)
Da Vinci's Daemons (Season 3 premiered on Starz Play instead of the Starz network - they are considered the same owner, so Starz Play wins)
Using the owner's premiere dates should be rare. When in doubt, always use the world premiere dates.