Harvard University
Harvard Smithsonian Center for Astrophysics: Time and the Calendar
This lesson helps learners discover the importance of time and the calendar, and how they each bring order to life. Each activity is designed for students understand the passage of time, how to keep time, international time zones,...
Wikimedia
Wikipedia: International Date Line
Wikipedia offers information on the International Date Line, an imaginery line that offsets the hours added as one travels east through time zones.
Wolfram Research
Wolfram Science World: International Date Line
This website provides a good general definition of the International Date Line.