title: How to change the units of difftime after computing the values, and not using units="xxx" when performing the calculations
body: <p>I am substracting dates. In the following example</p>
  
  <pre><code>bb
  
  FECHA_EFECTO_ESTADO FECHA_ANIVERSARIO_POLIZA
  9  2015-11-05 09:49:00               2015-11-05
  10 2015-11-05 09:51:00               2015-11-04
  </code></pre>
  
  <p>the columns are posixct date values. To create a new variable which is the difference of the other variables I could use:</p>
  
  <pre><code>library (dplyr)
  bb&lt;-aa&lt;-mutate(bb, day1=abs(FECHA_EFECTO_ESTADO-FECHA_ANIVERSARIO_POLIZA))
  bb
    FECHA_EFECTO_ESTADO FECHA_ANIVERSARIO_POLIZA            day1
  1 2015-11-05 09:49:00               2015-11-05  9.816667 hours
  2 2015-11-05 09:51:00               2015-11-04 33.850000 hours
  </code></pre>
  
  <p>By default, the units (days, hours, seconds) if the day1 variable depends on the amount of the difference. If I want to have the difference in days, I could do:</p>
  
  <pre><code>bb&lt;-mutate(bb, day2=abs (difftime(FECHA_EFECTO_ESTADO, FECHA_ANIVERSARIO_POLIZA, units="days" )))
  bb
    FECHA_EFECTO_ESTADO FECHA_ANIVERSARIO_POLIZA            day1           day2
  1 2015-11-05 09:49:00               2015-11-05  9.816667 hours 0.4090278 days
  2 2015-11-05 09:51:00               2015-11-04 33.850000 hours 1.4104167 days
  </code></pre>
  
  <p>IS there a way to specify the units (days in this case) <strong>after</strong> doing the calculations? I might find further down the analysis that I would prefer to have the difference in hours for instance, so:<br>
  <strong>How can I change the units of the day1 or day2 columns a posteriori?</strong><br>
  Thanks</p>