There are 60 minutes an hour. To determine the length of a quarter hour, divide 60 minutes by 4. The result is 15 minutes.
Therefore, each 15 minutes worked represents one quarter of one hour.
United States Department of Labor regulations specify that employers should round employee time to the nearest quarter hour. Employee time from 1 to 7 minutes may be rounded down and not counted as hours worked, but employee time from 8 to 14 minutes must be rounded up and counted as a quarter hour of work time.
Note that employers may be in violation of labor laws if they always round down and don't count a segment of employee time between 8 and 14 minutes as being a full quarter hour.