Number & Place
When calculating habit in menstruation (hayd) and purity (tuhr), a woman looks at both the number of days (‘adad) as well as the place in the month (zaman) she bled.
The Fiqh of It
‘Adad refers to the most recent number of days of valid blood (hayd) and valid purity (tuhr sahih)*. For example, Ayesha’s hayd was from September 1-5. Her following hayd was from October 1-5. Her ‘adad is 5 days for hayd and 25 days for tuhr.
Zaman refers to the due date of a woman’s next menstrual cycle, based on her purity from the previous month. In this case it is 5 days of hayd, preceded by 25 days of tuhr. (Manhal al-Waridin pp.180-4)
To really understand the interplay between ‘adad and zaman, let’s look at another example, Ayesha’s habit is 7 days for hayd preceded by 23 for tuhr. Accordingly, she was expecting her hayd to start on May 1, however it did not start until May 5. She then bled for 7 days.
The ‘adad remains 7 days for hayd, but increases from 23 to 27 days for tuhr. This changes the zaman of hayd as the date she is expected to begin bleeding has shifted forward by 4 days.
*A valid purity (tuhr sahih) is a purity that is 15 days or more, is not mixed with any bleeding in the beginning, middle or end and is preceded and followed by hayd.