@Jeffrey: it is because of two things: how the current time is count and the operating system and applications which handle time data (like Apache, MySQL, PHP).

You will find a very detailed (and understandable) explanation at Wikipedia "Year 2038 problem": http://en.wikipedia.org/wiki/Year_2038_problem

So the maximum time on 32-bit systems is 03:14:07 on Tuesday, 19 January 2038. Because a user of the plugin can type in a date after 19 January 2038 an error can occur. So in the plugin I set the maximum valid year value to 2037 to allow dates up to December 31, 2037 and to avoid a more complicated check for a date like "January 19, 2039".

Could that answer your question understandable?