Reference for Bava Batra 328:5

ומי בקי רבי במקושר והא ההוא מקושר דאתא לקמיה דרבי ואמר רבי שטר מאוחר זה וא"ל זונין לרבי כך מנהגה של אומה זו מלך שנה מונין לו שתים שתים מונין לו שלש

Was Rabbi, however, familiar with [the dating of] a folded [deed]? Surely, once a certain folded [deed] was brought<span class="x" onmousemove="('comment',' Lit., 'that came'. ');">¹³</span> before Rabbi who remarked, 'This is post-dated',<span class="x" onmousemove="('comment',' A year later than the current year. ');">¹⁴</span> and Zonin said to him, 'Such is the practice of this nation: [If a king] reigned a [full] year they count it as his second year;<span class="x" onmousemove="('comment',' Lit., 'two'. ');">¹⁵</span> [if] two [years], they count them as his third<span class="x" onmousemove="('comment',' Lit., 'three'. ');">¹⁶</span>

Jastrow