Siman 427
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א כְּשֶׁרֹאשׁ חֹדֶשׁ שְׁנֵי יָמִים, כּוֹתְבִין בַּשְּׁטָרוֹת בַּיּוֹם הָרִאשׁוֹן: בְּיוֹם רֹאשׁ חֹדֶשׁ שֶׁהוּא יוֹם שְׁלֹשִׁים לַחֹדֶשׁ שֶׁעָבַר. וּמוֹנִין לַחֹדֶשׁ מִיּוֹם הַשֵּׁנִי וְכוֹתְבִין בּוֹ בְּא' לַחֹדֶשׁ (טוּר). הַגָּה: כְּשֶׁמְעַבְּרִין הַשָּׁנָה כּוֹתְבִין בַּאֲדָר רִאשׁוֹן: אֲדָר, סְתָם, וּבַשֵּׁנִי: אֲדָר הַשֵּׁנִי (טוּר).
1 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."