_skillLevelToIndex = function( lvl ) {
  let idx = 0;
  if (String.is( lvl )) {
    lvl = lvl.trim().toLowerCase();
    const intVal = parseInt( lvl );
    if (isNaN(intVal)) {
      switch (lvl) {
        case 'beginner': idx = 1; break;
        case 'intermediate': idx = 2; break;
        case 'advanced': idx = 3; break;
        case 'master': idx = 4; break;
      }
    } else {
      idx = Math.min( intVal / 2, 4 );
      idx = Math.max( 0, idx );
    }
  } else {
    idx = Math.min( lvl / 2, 4 );
    idx = Math.max( 0, idx );
  }
  return idx;
}
Format a from/to date range for display.