options ls=75;

data one;
   input before after;
   subject=_n_;
   diff=after-before;
   cards;
12.3 12
12 12.3
12 12.5
13 12
13 13
12.5 12.5
11.3 10.3
11.8 11.3
11.5 11.5
11 11.5
11 11
11.3 11.5
;

proc ttest;
   title  "Good ole paired ttest";
   paired before*after;
run;

/* reconfigured data, one observation per 1 msmt, total of 2x12=24 */
data ftest;  
   set one;
   keep subject time period;
   array times{2} before after;
   array timename{2} $ ("before","after");
   do i = 1 to 2;
      time=times{i};
      period=timename{i};
      output;
   end;
run;
proc glm;
   title "proc glm to fit model for rcbd";
   class period subject;
   *model y=period subject;
   model time=period subject;  /* y changed to time (typo) */
run;
proc mixed;
   title "proc mixed to fit model for rcbd, subjects random";
   class subject period;
   model time=period;
   random subject;
   lsmeans period/diffs;
run;