string (($nano)+10**9*( (<seconds-since-midnight>) + 86400*((<day-of-year>)+365.24*(<4digit year>)) ) ) 
convert that into whatever normalized form is needed for the random num generator...

create array of your files w/a usecount field{init=0}, and a globcounter of all {0} fields; (so init, globcnt=sizeof(array);
then either on each execution, recount the arrary, or keep globcnt around (should be fine, as long as nothing updates array without
updating globcnt)

then gen random number (0..globcnt-1)

my fileindex=0;
while (newrand>0 && fileindex < (<count-of-file-in-array>) ) && globarray(fileindex)){++fileindex}
if (fileindex==sizeof(globarray) ) {
  error [.. note to log or whatever, probably don't bother user unless debugging, ];
  reinit_random_thing;
 if (alreadyfailed) {tell user very sorry, programmer is idiot, but probably ms's fault}
 else {
alreadyfailed=1; goto &self;}

display($globarray.filename[globarray.counter(fileindex)++]);