We classify "late" as arriving being over one minute late. This statistic is calculated using the time at which you press the "Enter" button on the site, so it won't be impacted by Zoom taking a while to open.
If, for example, you enter a lesson scheduled at 9pm at 8.59.59pm, but the Zoom meeting doesn't open until 8.01.32pm, you won't be marked as late!
Remember, though: if you're over 2 minutes late (for a 20 minute lesson) or 3 minutes late (for a 40 minute lesson) or your student has already left, this will count as a Tutor No-Show rather than an instance of Lateness.