Diablo II: Resurrected announcement: February 19th, 2021
Diablo II: Resurrected Single Player Technical Alpha start: April 8th/9th, 2021(7 weeks since announcement)
Diablo II: Resurrected Single Player Technical Alpha end: April 12th, 2021
Diablo II: Resurrected Multiplayer Stress Test Beta start(my guess): May 27th/28th, 2021(7 weeks since Single Player Technical Alpha start)
Diablo II: Resurrected Multiplayer Stress Test Beta end(my guess): May 31st, 2021
Diablo II: Resurrected Release Date(my guess): Tuesday, June 29th, 2021
That would give exactly one month to release after Beta ends! 