Reference for Niddah 145:27

יום אחד מנין

and I exclude the twelfth since it is not suitable for being counted as one of the seven clean days following the one that is deduced from 'or if she have an issue'.<span class="x" onmousemove="('comment',' It being the first day of menstruation. ');">²⁴</span>

Jastrow

Jastrow

Jastrow