[akshay@nio tmp]$ cat file
21:36:17
23:52:08
20:26:17
22:32:08

[akshay@nio tmp]$ cat s.awk
function dfor(time, a)
{ 
	  split(time, a, /:/); 
	  return 3600*a[1] + 60*a[2] + a[3] 
}
function hms(s, h,m)
{
	  h=int(s/3600);
	  s=s-(h*3600);
	  m=int(s/60);
	  s=s-(m*60);
	  return sprintf("%02d:%02d:%02d", h, m, s);
}


FNR==1{ print "Start","END","Seconds","Minutes","HH:MM:SS" }

{ 
	s[v = (FNR%2)?"s":"e" ] = $1	
	if(v=="e")
	{
		seconds = dfor(s["e"])-dfor(s["s"])
		print s["s"],s["e"],seconds,seconds/60,hms(seconds)
	}
}

[akshay@nio tmp]$ awk -f s.awk file
Start END Seconds Minutes HH:MM:SS
21:36:17 23:52:08 8151 135.85 02:15:51
20:26:17 22:32:08 7551 125.85 02:05:51