The most likely reason is that your client's Google calendar is set to the incorrect time zone.
If a client with a Gmail address complains that they received a confirmation or reminder email with a incorrect start time, this is most likely due to a Google calendar event notification that appears above the Schedulista email that looks like this...
The notice isn't actually coming from Schedulista nor is it part of the email. It is automatically generated by Google, allowing Gmail users with Google calendars to easily add events to their Google calendar.
In the example above, the issue is that the client's Google calendar is set to a different time zone. The client scheduled for an appointment in the US Pacific time zone but their Google calendar is set to an Irish time zone (IST).
If the client looks at the actual email, they will see that the appointment time will be at the correct time, in the correct time zone. Your client can set their Google calendar to the correct time zone by following these instructions.