Here's an example:
With your example that will give you 15 rather than 65. I don't know where you expect 65 to come from. If you want the total number of seconds (1 min 15 secs = 75 secs), you don't want seconds-from-duration (which will give you 15), you want to do
xs:dayTimeDuration(preview_start_time) idiv xs:dayTimeDuration('PT1S')
Remember that you need an XSLT 2.0 processor and that in the above examples you need to declare the xs namespace.
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference