Turei Even states the same idea without mentioning the Rambam. He cites proof to this concept from the intercalation of the year, where Beis Din can not add a new month after Adar but they can decide to add an additional month to the year before Adar and even immediately after Tishrei.
Reb Itzele in Zecher Yitzchok (11) questions the Turei Even’s proof. When Beis Din decides that there should be another Adar this year, they are not ruling on the future, rather they are deciding that this year should be a leap year and not a regular year. Sanctifying the upcoming month on the twenty-ninth day of the previous month is considered as ruling on the future since the preceding month has no association to this month.
The Zecher Yitzchok explains the Rambam differently. The reason Beis Din can sanctify the new month on the twenty-ninth is not because they can rule regarding the future but rather it is because they are deciding on the present. Sanctifying the new month is in fact deciding on how many days are contained in the previous month. They can decide on the twenty-ninth day that the thirtieth day will be Rosh Chodesh because in essence they are determining that this month will be comprised of twenty-nine days.
It emerges (and this can be found in sefer Poseach Shaar) that theoretically Beis Din can sanctify the new month anytime before Rosh Chodesh. Practically, it can only transpire on the twenty-ninth since the new moon is not visible until then. This can be relevant to the times that there is no Beis Din and the months are decided through calculation.
Reb Chatzkel Abromsky inquires as to what is the difference between sanctifying the new month and intercalating the year. Why can the sanctifying of the new month take place prior to the new month and the intercalating of the year can only transpire during that year? This is answered according to the Zecher Yitzchok. Beis Din, in sanctifying the new month, is not ruling on the future, rather they are deciding how many days are in the present month. Intercalating the year is a ruling which is only relevant to this year and it cannot be decided on in the preceding year.
Reb Meir Simcha cites proof to this novelty from the Yerushalmi that states the reason for not lighting the torches in Tishrei because it constitutes a desecration of Yom Tov. The halacha in Tishrei is that the messengers cannot be sent out until they hear Beis Din proclaim “It is sanctified.” It is evident from here that they heard Beis Din proclaim on the twenty-ninth that the following day will be Rosh Chodesh.