I have a requirement where the User's timezone might be in EST,PST or IST. Whenever an event is created for an Account, Start time of the event will be captured in the USER's timezone. But, there will be a formula field on an Account which specifies Timezone of the user (say, GMT-8 Pacific) and User's timezone and the formula field timezone will not be same. On event, i need to capture exact value of Start time based on the formula field on Account.

For example : User's timezone is EST. Formula field on Account is in IST. Say the user creates an Event at 10:30 AM ( which is 10:30 AM EST) . The new field on Event should have a value of 8 PM ( which is 8 PM IST) .

Kindly help me on how to resolve this issue in apex.