Yeah... I didn't think I was confused... until now...
6 Hours of the Glen is definitely an IMSA (essentially NASCAR) sanctioned race in real life that will take place on July 1st, 2018.
The iRacing Endurance Le Mans Series (iELMS based off the ACO-sanctioned World Endurance Championship) will be wrapping up Season 2 at Watkins Glen with a 6-hour race on June 2nd-3rd, 2018.
Those are a month apart, so I'm not sure we can say those events are intentionally scheduled that way for one to emulate the other. Unless, of course, you're a GTE driver, in which case 6 hours of dodging prototypes around Watkins Glen is 6 hours of dodging prototypes around Watkins Glen... But the GTE class is the only class 100% homologated between NASCAR's IMSA and ACO's WEC.