* Based on my above suggestion, I'm providing code for you.
Just tell the client "Set every event Start/End time based on your own timezone". Then add that three-letter tag to the CSS.
Live site: https://www.oliversacksdoc.com/events Using "MOJAVE", the 7.0 template (based on Brine).
You can see below (in my screenshot) where each of these elements will appear.
.eventlist-datetag span.event-time-12hr::after {
content: "EST";
padding-left: 4px;
font-size: 12px;
font-weight: bold;
}
.event-time-12hr-end::after {
content: "Timezone: EST";
padding-left: 10px;
font-size: 16px;
font-weight: bold;
}