See this Comm Center message by Tim Long. Extend these ancient data validation limits to 2040.
See http://forums.dc3.com/showthread.php?12577-Odd-repeating-error-when-editing-a-plan-in-the-web-schedule-browser&p=77945#post77945
Also see Eric Dose’s suggestion to make the date validation based on current time. May not be possible with validation library but do look!! Decided no due to hacking the in-browser Javascript validation library. Risky.