Considering one day of the year to be the entire year, in calculating the growth of a plant for its status vis-a-vis prohibitions on using produce of a tree's first 3 years: Rosh HaShanah 10a-b
Counting the remainder of the 1st day when a menstruating woman sees blood, as the 1st day of her 7-day period of waiting: Rosh HaShanah 10a
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