To ensure that everyone with a membership is given their full first interval, and is not shorted up to a day based on when the membership was purchased, some memberships' first intervals are up to a day longer than the interval is configured. We call this "Day 0."
Example: A customer purchases a 2-week (14-day) unlimited, non-recurring membership at 9 pm local time on Sunday, March 1st. From the time the membership was purchased to the end of the day, Sunday, 9:00 pm - 11:59:59 pm, is considered "Day 0." This means that Monday, March 2nd at 12:00:00 am local time would be the start of "Day 1" of their membership. The customer can continue to use the membership for 14 days. They can use their membership through 11:59:59 pm local time on March 15th. The total duration of this membership was 14 days, 2 hours, 59 minutes, and 59 seconds.
Here are the memberships that will have a Day 0
Memberships with a Daily or Weekly Interval that start on purchase - The date of purchase is Day 0 and the following day is considered Day 1 of the interval.
Here are the memberships that do NOT have a Day 0:
Monthly and Yearly Memberships
Memberships that start on specific dates
Memberships that start on first use - The date of the first use is Day 1 and the following day is considered Day 2 of the interval.