I asked the same question during the PSS fs9 era. The conclusion was the PSS T7 FMC was not fully functional as the real one. Now, we have the PMDG 777, I still have the same question. How can add the international dateline into FMC and have it display on the ND? I captured what I saw from justplane DVD (the Air canada 777-300er one), is it just a fix at W180?

 

That is just a waypoint... the dateline has little meaning to the flight, which most operators if not all use UTC as the date/time where the date is the same globally..  Also, the dateline doesn't follow the 180 meridian as you know.