INT2SECARR takes a structure (e.g., roadmap or index)
containing time and will create an array of times (t) in
seconds from some given reference. If the second parameter is not passed,
then the first time in the input array is used
IDL> t = int2secarr(roadmap)
The example above will produce an array of times with respect to the first time.
IDL> t = int2secarr(roadmap, ref_time)
IDL> t = int2secarr(roadmap, '1-sep-91')