Purity: Meaning of ‘One Month’

Practical Laws of Islam as per the teachings of Ayatullah Sistani

Ruling 491:

The meaning of ‘one month’ – except with regard to determining the habit of time – is the passing of thirty days from the start of bleeding and not from the first day of the month to the last. With regard to determining the habit of time, the lunar month is intended, not the solar.

-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)