A woman's cycle is measured from the first day of her menstrual period until the first day of her next period. The average woman's cycle normally is between 28-32 days, but some women can have much shorter cycles or much longer ones.
Ovulation can be calculated by starting with the day the last menstrual period starts or by calculating 12-16 days from the next expected period. Most women ovulate anywhere between Day 11 - Day 21 of their cycle, counting from the first day of the last period. Ovulation can occur at various times during a cycle, and may occur on a different day each month.
Most books and articles, and posts I have read seem to agree that it's 7-10 days before and 7-10 days after