how to determine luteal phase
Taking this into account, you can calculate the time of ovulation within your cycle by subtracting the length of your luteal phase from the length of your cycle. For example, if your cycle is 28 days long and your luteal phase is 12 days long, the ovulation will occur on day 16 of your cycle (28-12=16). Ovulation Calendar uses this formula to calculate your time of ovulation.