Shulchan Arukh, Orach Chayim Siman 427
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כשראש חדש שני ימים כותבין בשטרו' ביום הראשון ביום ראש חדש שהוא יום שלשים לחדש שעבר (ומונין לחדש מיום השני וכותבין בו בא' לחדש) (טור): הגה כשמעברין השנה כותבין באדר ראשון אדר סתם ובשני אדר השני: (טור):
How documents are dated when Rosh Chodesh falls on two days. Contains one section When Rosh Chodesh falls on two days, documents written on the first day are dated "on Rosh Chodesh" which is the thirtieth day of the previous month. Rema: The month is counted from the second day and documents written on it are dated the first of the month. When the year is intercalated, the first Adar is written simply as "Adar" and the second is written as "Adar the Second."