In an iCal event, event time is not changing for invitees based on their time zone. Why?

I'm on an iMac with iOS 10.5.5. I created an event and invited others to it. It's my understanding that the event time should adjust to the recipient's time zone, but I'm being told that isn't happening. An event created with a 12 pm EST start time is coming through as 12 pm start time for someone in PST time zone. She has checked her date and time settings and they are correct for her time zone. How to fix this?