title: How to subtract or add date from a variable?
body: <p>I have a date in the variable <code>x=20170402</code>, getting this value from another file.<br>
  I want to modify this by adding/subtracting and save to new variable. How can i do this?<br>
  ex: if i subtract one day, <code>y=20170401</code>; two days, <code>y=20170331</code><br>
  and it is GNU based.</p>