Commentary for Berakhot 60:11
אמר אמימר מסתברא מילתא דרב בחדש מלא אבל בחדש חסר מחזירין אותו
Amemar said : It is probable that Rab's statement applies to a full month ; but in the case of a detective month, we require him to repeat it.
Abraham Cohen Footnotes to the English Translation of Masechet Berakhot
The month is "full" when it has 30 days, "defective" when it has 29. When the moon is "full," two days (the concluding day of the old and the first of the new) are observed, and the extra prayer said on both. Therefore if he omitted it one evening, he can say it the next.
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