Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="age-0-days-or-less">zero</div>
<div class="age-7-days-or-less">week or less</div>
<div class="age-14-days-or-less">two weeks or less</div>
<div class="age-31-days-or-less">a month or less</div>
<div class="age-60-days-or-less">two months or less</div>
<div class="age-90-days-or-less">three months or less</div>
<div class="age-365-days-or-less">a year or less</div>

SassMeister-input.scss

// ----
// Sass (v3.3.8)
// Compass (v1.0.0.alpha.19)
// ----

$cutoffs: (1: 0,2: 7,3: 14,4: 31,5: 60,6: 90,7: 365, 8:3650);
$opacity-factor: (length($cutoffs)/ 80 );

@mixin background-opacity($color, $opacity: 0.3) {
    background: $color; /* The Fallback */
    background: rgba($color, $opacity);
}

@mixin primary-color($number, $opacity) {
  
  $red: red;
  $blue: blue;
  
  @if $number < 1 {
    $color: $red;
    @include background-opacity($color, $opacity);
//    text-shadow: 1px 1px 0 contrast-color( contrast-color($color));
  } @else {
    $color: $blue;
    @include background-opacity($color, $opacity);
    text-shadow: 1px 1px 0 contrast-color( contrast-color($color));
  }
}

@each $index, $cutoff in $cutoffs {
   .age-#{$cutoff}-days-or-less {
    @include primary-color($cutoff, $opacity-factor * $index);
   } 
}

SassMeister-output.css

.age-0-days-or-less {
  background: red;
  /* The Fallback */
  background: rgba(255, 0, 0, 0.1);
}

.age-7-days-or-less {
  background: blue;
  /* The Fallback */
  background: rgba(0, 0, 255, 0.2);
  text-shadow: 1px 1px 0 contrast-color(contrast-color(blue));
}

.age-14-days-or-less {
  background: blue;
  /* The Fallback */
  background: rgba(0, 0, 255, 0.3);
  text-shadow: 1px 1px 0 contrast-color(contrast-color(blue));
}

.age-31-days-or-less {
  background: blue;
  /* The Fallback */
  background: rgba(0, 0, 255, 0.4);
  text-shadow: 1px 1px 0 contrast-color(contrast-color(blue));
}

.age-60-days-or-less {
  background: blue;
  /* The Fallback */
  background: rgba(0, 0, 255, 0.5);
  text-shadow: 1px 1px 0 contrast-color(contrast-color(blue));
}

.age-90-days-or-less {
  background: blue;
  /* The Fallback */
  background: rgba(0, 0, 255, 0.6);
  text-shadow: 1px 1px 0 contrast-color(contrast-color(blue));
}

.age-365-days-or-less {
  background: blue;
  /* The Fallback */
  background: rgba(0, 0, 255, 0.7);
  text-shadow: 1px 1px 0 contrast-color(contrast-color(blue));
}

.age-3650-days-or-less {
  background: blue;
  /* The Fallback */
  background: rgba(0, 0, 255, 0.8);
  text-shadow: 1px 1px 0 contrast-color(contrast-color(blue));
}

SassMeister-rendered.html

<div class="age-0-days-or-less">zero</div>
<div class="age-7-days-or-less">week or less</div>
<div class="age-14-days-or-less">two weeks or less</div>
<div class="age-31-days-or-less">a month or less</div>
<div class="age-60-days-or-less">two months or less</div>
<div class="age-90-days-or-less">three months or less</div>
<div class="age-365-days-or-less">a year or less</div>