FullCalendar is an open source jQuery plugin for creating customizable drag and drop event calendars, with 7500 stars on GitHub. Its creator, Adam Shaw, encountered numerous bugs related to Daylight Savings Time (DST) in the first two years of development, including issues with date arithmetic, time zone changes, and DST-related problems that could cause the calendar to freeze or display incorrect dates. To address these issues, Shaw rewrote his internal system and switched to using a third-party library called XDate, which was later replaced by Moment.js, allowing him to focus on other aspects of the project and avoid maintenance responsibilities for date functionality.