Suppose a user of your website enters a date range.
2009-1-1 to 2009-1-3
You need to send this date to a server for some processing, but the server expects all dates and times to be in UTC.
Now suppose the user is in Alaska or Hawaii or Fiji. Since they are in a timezone quite different from UTC, the date range needs to be converted to something like this:
2009-1-1T8:00:00 to 2009-1-4T7:59:59