It literally says May in the official post
https://www.project1999.com/forums/s...d.php?t=333743
The problem is the wiki referenced the timeline oddly and has changed:
http://wiki.project1999.com/Timeline#Patch_Notes
The first table shows Hate releasing in June... however the second table "Development timeline" shows Hate release in May.
If you look at the live patch notes and count the number of months after Fear was released you'd get May is when Hate releases (3 full months of Fear release which is Feb, March, April -> Release in MAY).
http://wiki.project1999.com/Patch_Notes
"July 26th, 1999 Plane of Fear launched" -> Round up to August to make math simple (February in Green Timeline)
"November 3, 1999 Plane of Hate opens" -> November (May in Green Timeline)
So yes the official post, wiki, and actual math for when it came out on live were all disconnected at some point, but none of them say April now.