Smart Dates is a module about user interface, and making your Drupal site more magical.
If you've ever had to create a lot of content with Dates included, you'll have run into a standard frustrating user experience. From Date and To Date are populated with some defaults, and after changing the From Date, you have to go and also change the To Date. It's aggravating, as we've come to expect our date fields to be smart, and do expected smart things.
Smart Dates solves that, using datejs. It links From and To, Dates and Times together with javascript. Maintaining intervals between them.
Algorithm
For the Time field, the logic is as follows: if From and To started off the same (set by some default) make the To date 2hrs after the From date (a setting for this default will be added later). After that it does interval math between From and To, so if From is 6:30PM and To is 8:00PM, and you change From to 7:00PM, To will now be 8:30PM.
For the Date field, intervals are always what's used. If you move From ahead by a week, To date will shift the same amount.
This should work regardless of what input formatting you use for dates and times, as the module translates between the date format you have specified for the date field into the javascript formatter.