Sanctification of the new moon when the Temple is rebuilt

Q: Will they return to sanctifying the new moon by the testimony of witnesses when the Temple is rebuilt?
A: Yes, when the Sanhedrin is reconstituted. But when the Sanhedrin sanctifies the new moon, they also take the calendar into consideration. You can see this in the Laws of Sanctification of the New Moon of the Rambam. For example, if witnesses arrive, they examine them, and if they truly saw the new moon, the Sanhedrin fixes the new month. But if no witnesses arrive, there will not be a Rosh Chodesh?! Of course, there will be. How will they accomplish this? By the calendar. This means that the sanctification of the moon will be comprised of sightings and the calendar.