If you are conducting online studies, then you will often be scheduling across multiple timezones. This means that the time you invite and schedule participants may be different from the time they display on your calendar—which will be in your local time.
This can be confusing. But, to be sure, you can check their timezone against your own. You can find your timezone in the top left of your calendar. You can find the participants timezone under their name by clicking on their time slot.
For in-person studies, make sure to lock the timezones. More on that here