title: Converting Date-Time (yyyy-mm-dd hh:mm:ss) format to seconds in EXCEL
body: <p>I have a long list of "date-time" data which are formatted as (yyyy-mm-dd hh:mm:ss) and sorted in chronological order.
  What I want to achieve is setting the first "date-time" as "0s" and set the succeeding "date-time" with reference to "0s".
  For example, if the first date is 2015-01-01 00:00:00 and nth date is 2015-01-02 00:00:00, then since exactly 24 hours=86400 seconds passed, I want the "date-time" to be converted to "0" for the first datum and "86400" for the nth datum. I don't want them to be renewed when the day passes.
  I tried to achieve this by simply subtracting the 1st datum form the nth datum, but I confronted the problem of seconds getting renewed when the day pass. I need some concrete hint about solving this problem or if there is simple function for achieving this, please let me know. </p>