Vrshikhar Shikhar refactored the Busy-times API in the calcom/cal.com repository, focusing on modernizing timezone handling to improve validation and documentation while maintaining backward compatibility. Using TypeScript and leveraging backend and API development skills, he replaced the deprecated loggedInUsersTz parameter with a new timeZone parameter, ensuring at least one timezone is always provided through a custom validation constraint. He centralized timezone normalization logic for consistency and updated endpoint behavior to prioritize the new parameter. The work included comprehensive updates to API documentation and end-to-end tests, resulting in clearer API consumption and a more reliable integration experience for future enhancements.