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The ideal way of deciding the day of Rosh Chodesh is for Beis Din to declare it after witnesses come and say they saw the new moon. The new moon is only visible around sunset, so usually the witnesses have almost 24 hours to get there in time to testify.

Let's say witnesses see the new moon in Beijing on Sunday night, where (at least in the summer) sunset is around the same local time as in Yerushalayim, or 5 hours earlier in absolute time. Using the plane that currently holds the record for fastest airspeed, they can fly to Tel Aviv in less than 2 hours and get to Yerushalayim in plenty of time to testify before sunset on Sunday.

Can Beis Din declare Rosh Chodesh on Sunday, even though when the witnesses saw the moon it was already Monday for them?

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Rambam, Kiddush haChodesh, 2:9

ראוהו בית דין עצמן, בסוף יום תשעה ועשרים--אם עדיין לא יצא כוכב בליל שלושים--בית דין אומרים מקודש מקודש, שעדיין יום הוא. ואם ראוהו בליל שלושים, אחר שיצאו שני כוכבים--למחר מושיבין שני דיינין אצל אחד מהם, ויעידו הם השניים בפני השלושה; ויקדשוהו השלושה.‏. If the court itself saw [the new moon] at the end of day 29 -- if a star hadn't come out yet on the night of the 30th -- the court declares it holy, as it is still day. ...

So, yes the court can declare day 30 as Rosh Chodesh even on day 29 if they have testimony about the upcoming night's moon.

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