When working with dates and times in web applications, time zones introduce several common challenges. For instance, users in different locations may see different local times for the same event, and storing dates in one time zone while displaying them in another can lead to confusion and errors. Day.js addresses these challenges by offering plugins such as utc and timezone. These plugins let you parse, convert, and display dates in any IANA time zone, making it much easier to build applications that are consistent and user-friendly across the globe. Using these plugins, you can ensure that your application handles daylight saving time changes and time zone conversions reliably, which is essential for internationalization and accurate scheduling.