The Nintendo NX, which is still a codename, will be releasing during the end of first quarter of next year.
The March 2017 release window comes from a late-night tweet from Nintendo, seen above, which says the next generation of Nintendo arrives March 2017 with the hashtag, #NX. Additional details, like the design and its true name, are still a mystery.
Nintendo also revealed tonight that the next Zelda will launch on both Wii U and NX next year. You can read more about that here.
"The next generation of Nintendo" leaves some room for interpretation as it does not specifically mention a console, or hardware. I think we were all hoping for a November 2016 release, so this new window is a bit of a surprise.