SQL - Difference between odd/even columns into ano

I have a table which consists of the following data.

 Employee      TimeRegister1           TimeRegister2           TimeRegister3           TimeRegister4           
    77    2014-04-01 11:51:43.000 2014-04-01 14:03:52.000 2014-04-01 14:17:01.000 2014-04-01 16:01:12.000
    77    2014-04-03 09:48:33.000 2014-04-03 12:13:43.000 2014-04-03 12:22:27.000 2014-04-03 14:03:43.000
    181   2014-04-02 08:24:34.000 2014-04-02 13:01:10.000 2014-04-02 14:30:31.000 2014-04-02 18:04:04.000

What I need is to write, on another column, the total calculated from the differences between each pair of columns (odd minus even) for each employee/day.

In the example above, for Employee 77 and 2014-04-01, it should write on another column the sum of the differences between TimeRegister 2 - TimeRegister 1, TimeRegister 4 - TimeRegister 3.

Something like this should be be output (seconds are irrelevant, just need hours/minutes):

 Employee      TimeRegister1           TimeRegister2           TimeRegister3           TimeRegister4              CALCULATEDCOL        
    77      2014-04-01 11:51:43.000 2014-04-01 14:03:52.000 2014-04-01 14:17:01.000 2014-04-01 16:01:12.000    2014-04-01 03:56:00.000

Besides the TimeRegister Columns can be up to 30 (I just showed 4 columns but could be more) so I need a sequenced calculation for each odd/even pair until it runs out of columns.

Any help how I can achieve this in SQL will be greatly appreciated. Thanks.

Assuming that the table presented is correct (i.e. not 30 columns), you can do this by DATEDIFFing the DateTimes, add the differences together, and then adding these back to the base date of one of the times (assuming all DateTimes are on the same day). You can eliminate the seconds by choosing minutes as your diff / add resolution. And to handle missing data, you would require ISNULL() or COALESCE on each In / Out pairing.

SELECT Employee, TimeRegister1, TimeRegister2, TimeRegister3, TimeRegister4, 
       DATEADD(mi, ISNULL(DATEDIFF(mi, TimeRegister1, TimeRegister2), 0) 
                 + ISNULL(DATEDIFF(mi, TimeRegister3, TimeRegister4), 0),
               CAST(CAST(TimeRegister1 AS DATE) AS DATETIME)) 
       as CALCULATEDCOL        
FROM TimeRegister;

As per the comment, it may be better to split the CALCULATEDCOL into separate date and time components, since as it stands, CALCULATEDCOL doesn't represent a point in time at all.

SqlFiddle here

If you truly have up to 30 columns, you would need to repeat the calculation for each pair of In/Out data (and hope that employees don't leave the building more than 15 times / day!).

Edit
To do this from a raw table of clock in / out data, assuming that the direction is obtainable (otherwise the data is next to useless), and discarding any data where the employee hasn't clocked in and out consecutively, on the same day, how about:

WITH cte AS 
(
   SELECT Employee, 
          TimeRegister,
          Direction,
          CAST(TimeRegister AS DATE) AS TheDate,
          ROW_NUMBER() OVER (PARTITION BY Employee, CAST(TimeRegister AS DATE) 
                             ORDER BY TimeRegister ASC) AS Rnk
   FROM TimeRegister
)
SELECT 
   cur.Employee,
   DATEADD(mi, SUM(DATEDIFF(mi, cur.TimeRegister, nxt.TimeRegister)), 
           CAST(cur.TheDate AS DATETIME)) AS CALCULATEDCOL
   FROM cte cur
      INNER JOIN cte nxt
      ON cur.Employee = nxt.Employee 
         AND cur.TheDate = nxt.TheDate AND cur.Rnk + 1 = nxt.Rnk
   WHERE cur.Direction = 'I' AND nxt.Direction = 'O'
   GROUP BY cur.Employee, cur.TheDate;

SqlFiddle here