When you use the Contacts database to schedule a "broadcast email" for one or more Contacts, and schedule it later that day or in the future, we use the time zone of the computer making the request. We look at the IP address you log in from, and calculate the time zone of that IP address using the Internet's "geo-location" function.
Only broadcasts created from the Contact database are handled this way. In all other cases, such as when you schedule the time for an email in a Sequence, your account's time zone is used. Your account's time zone is set by your Administrator in their profile.