Calculate event offsets in js

Summary: Ref T8300, Calculate event offsets in js

Test Plan: no user facing changes

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T8300

Differential Revision: https://secure.phabricator.com/D12981