title: SPSS: How to compute duration between two points of time (without full date information)
body: <p>In a study of mine, participants gave me the time they </p>
  
  <ol>
  <li>usually go to bed (bedtime) and </li>
  <li>usually get up from bed (outofbed) (format is hh:mm).</li>
  </ol>
  
  <p>I now want to compute a new variable indicating the duration between these two points of time.</p>
  
  <p>I already used SPPS' inbuilt TimeWizard and the function <code>newVar = CTIME.HOURS(outofbed-bedtime)</code>. This does not give me correct results for bedtime hours before 24.00 (since e.g. 23.00-7.00 = 16h, instead of 8h). </p>
  
  <p>I thoroughly used my googlefoo skills, but all answers I found used information on both date AND Time (eg. dd:mm:hh:mm). I expect the problem is that SPSS internally saves all datetime information as seconds since the beginning of the gregorian calendar. Since I lack the information of the date, SPSS does not have absolute values with which it can calculate. At least, this is my assumption so far.</p>
  
  <p>I hope you know a solution to my problem.
  Cheers, Dekay</p>