title: How to replace parts of a text with evaluation result
body: <p>I want to replace each occurrence of <code>@(some str)</code> in a string with the result of <code>date --date="some str" +%s</code>. Where 'some str' can be anything that is passed as is to the --date argument. Note that I want to support several <code>@()</code> occurrences.</p>
  
  <p>For example, the string <code>from @(-1 week) to @(-1 day)</code> should become <code>from 1435830929 to 1436349343</code>. And if I evaluate it in one week, it should be <code>from 1436435800 to 1436954213</code></p>