How to align this span to the right of the div?

The solution using flexbox without justify-content: space-between.

<div class="title">
  <span>Cumulative performance</span>
  <span>20/02/2011</span>
</div>

.title {
  display: flex;
}

span:first-of-type {
  flex: 1;
}

When we use flex:1 on the first <span>, it takes up the entire remaining space and moves the second <span> to the right. The Fiddle with this solution: https://jsfiddle.net/2k1vryn7/

Here https://jsfiddle.net/7wvx2uLp/3/ you can see the difference between two flexbox approaches: flexbox with justify-content: space-between and flexbox with flex:1 on the first <span>.

An alternative solution to floats is to use absolute positioning:

.title {
  position: relative;
}

.title span:last-child {
  position: absolute;
  right: 6px;   /* must be equal to parent's right padding */
}

See also the fiddle.

Working with floats is bit messy:

This as many other 'trivial' layout tricks can be done with flexbox.

   div.container {
     display: flex;
     justify-content: space-between;
   }

In 2017 I think this is preferred solution (over float) if you don't have to support legacy browsers: https://caniuse.com/#feat=flexbox

Check fiddle how different float usages compares to flexbox ("may include some competing answers"): https://jsfiddle.net/b244s19k/25/. If you still need to stick with float I recommended third version of course.

If you can modify the HTML: http://jsfiddle.net/8JwhZ/3/

<div class="title">
  <span class="name">Cumulative performance</span>
  <span class="date">20/02/2011</span>
</div>

.title .date { float:right }
.title .name { float:left }