As a rule of thumb, always pass dates to Smarty as timestamps. This allows template designers to use date_format for full control over date formatting, and also makes it easy to compare dates if necessary.
NOTE: As of Smarty 1.4.0, you can pass dates to Smarty as unix timestamps, mysql timestamps, or any date parsable by strtotime().
When using {html_select_date} in a template, The programmer will most likely want to convert the output from the form back into timestamp format. Here is a function to help you with that.