setDisplayValue() does not return anything, so you are setting your input variables to null.

Try getDisplayValue() instead:

var tz = Packages.java.util.TimeZone.getTimeZone("GMT");
var sd = new GlideDateTime();
sd.setTZ(tz);
var st_dte= sd.setValue(current.start_date);
var frmtdt = 'yyyyMMdd HH:mm:ss';
sd.setDisplayValue(st_dte,frmtdt);
variable.inputs.planned_start_date = sd.getDisplayValue();

var ed = new GlideDateTime();
ed.setTZ(tz);
var end_dte= ed.setValue(current.start_date); // this was sd, i think you meant ed
var end_frmtdt = 'yyyyMMdd HH:mm:ss';
ed.setDisplayValue(end_dte,end_frmtdt);
variable.inputs.planned_end_date = ed.getDisplayValue();