A minimal solution is as follows:

Constant SUNRISE  360;  ! i.e., 6 am
Constant SUNSET  1140;  ! i.e., 7 pm
Attribute outdoors;     ! Give this to external locations
Attribute lit;          ! And this to artificially lit ones
Global day_state = 2;
[ TimePasses f obj;
  if (the_time >= SUNRISE && the_time < SUNSET) f=1;
  if (day_state == f) rfalse;
  objectloop (obj)
  {   if (obj has lit) give obj light;
      if (obj has outdoors && obj hasnt lit)
      {   if (f==0) give obj ~light; else give obj light;
      }
  }
  if (day_state==2) { day_state = f; return; }
  day_state = f; if (location hasnt outdoors) return;
  if (f==1) "^The sun rises, illuminating the landscape!";
 "^As the sun sets, the landscape is plunged into darkness.";
];