The other day I came across this little bug in Magento 1.1.6. After setting the products special price and clicking Save and continue Edit, then after adding some additional input to product and clicking the Save and continue Edit i got the following error Undefined index year… in Date.php on line 2614.
If you look at the screenshot you can see that there is a bug on the Prices tab, where upon each Save and continue Edit, Magento does not pool the year value into the Special Price From Date and Special Price to Date fields. Its missing the middle year value. So basically what you need to do is to set it up upon each Save and continue Edit.