Counting the remainder of the 1st day when a menstruating woman sees blood, as the entire 1st day of her 7-day period of waiting: Rosh HaShanah 10a; Niddah 6a
How to count after discovery of a menstrual bloodstain: Niddah 6a
Counting part of the 7th day after a menstruating woman sees blood, as the entire 7th day to complete the period of waiting: Rosh HaShanah 10a