Apparently product_sale_start dates are handled according to timezones while the other dates (created, modified) are not.
After setting the product_sale_start to the default time entered by the calendar control, here is what I get in the database
FROM_UNIXTIME(`product_created`) FROM_UNIXTIME(`product_modified`) FROM_UNIXTIME(`product_sale_start`) NOW()
2012-12-17 17:18:34 2013-01-25 09:21:31 2013-01-25 01:21:00 2013-01-25 09:21:35
What is the rationale for handling product_sale_start differently than everything else?