For your Duration_seconds try :
(Time(End_Time) - (Start_Time)) * 24 * 60 * 60 as Duration_seconds
A timestamp is held as the number of days since a certain date ages ago. The integer part of it is the number of days since then and the decimal part is the fraction of a day. It is held as a dual and the text representation is the standard human readable format.
The 'ss' argument to your interval() formats the dual text representation to be human readable seconds, but the dual number part remains as a decimal.
So to get the number of seconds multiply the decimal by the number of seconds in a day, which is 24*60*60..