You don't actually express time as days, hours, minutes and seconds the way
you have shown it.  It is only hours, minutes and second

Assuming the time is in cell A1 *and* has zero days which can be ignored:

=RIGHT(A1,LEN(A1)-FIND(":",A1))*24*60*60    format as General

If you do have data in the format shown with days as the first element, you
could use:

=LEFT(A1,FIND(":",A1)-1)*24*60*60+RIGHT(A1,LEN(A1)-FIND(":",A1))*24*60*60

or

=(LEFT(A1,FIND(":",A1)-1)+RIGHT(A1,LEN(A1)-FIND(":",A1)))*24*60*60

Regards

Trevor

Times are stored internally by Excel as fractions of a 24-hour day, so
that 12:00:00 would be stored as 0.5 and 18:00:00 as 0.75.
Consequently, to convert a time to seconds, you must multiply by the
number of seconds in a day - it is easier to remember this as *24*60*60
rather than the actual number (86400).

Hope this helps.

Pete