Adding Timezone Support to a production app

Hi there, I have an app with users in one timezone with the date / time stored in their location. Now there are more users in more locations and I would like to support them - ideally without changing to historical data. At the moment the app uses a time offset field to allow two people to see one note expressed in their timezone. There are two use cases for this one is the free/busy calendar, the other is the meeting invite ICS file. At the moment I would like to support ICS which are GMT based. My guess is that: * Assign each user to their own timezone * I take the diary row ($start_date, $start_time) and convert it to a GMT timestamp * Make the ICS file I am keen to hear of better solutions or ideas. Cheers P