Slipspace isn't instant. I agree with the fourth post. CE would've begun a few days after the end of Reach.
Let's keep the answers simple, ok? 2552. there.
To contradict my previous answer, (the very first one), Halo Reach ended on August 30, 2552. Halo CE began on September 19, 2552, three weeks after Halo Reach ended.
The earliest known date is July 21, (I think) 2552 before it's ensuing battle and eventual fall. The latest was August 30, 2552.
Some other game, we'll go back to Reach...2589 when they will recolonize after possibly Halo 6 (possible myth)...since 3 games fitted into almost 3 months...really badass for a Master Chief Petty Officer Spartan...and his Sangehellii companion...