0 - 2 of 2 tags for seconds

I have a timestamp with time zone field and I would like to get the number of seconds that have elapsed from the start of that day.  This seems like it should be very trivial but I've searched for hours and tried many things that don't work.  What is a simple method for getting the number of seconds?

Hi, i have two timestamp columns of 2 fractional seconds precision,

i want to take the difference of these 2 timestamps and then have interval upto 0 fractional seconds only
 

For 2 fractional seconds Interval: