Getting a relative timestamp from timestamp stored in a file

Shell Programming and Scripting
1

Hi,
I've a file in the following format

1999-APR-8 17:31:06     1500     3     45
1999-APR-8 17:31:15     1500     3     45
1999-APR-8 17:31:25     1500     3     45
1999-APR-8 17:31:30     1500     3     45
1999-APR-8 17:31:55     1500     3     45
1999-APR-8 17:32:06     1500     3     45

As you can see the file is sorted according to the timestamp.
I want the output in such a form that it will consist of relative timestamp in seconds for every timestamp in each line stored in a file.
For eg:
For above file the relative timestamp will be calculated w.r.t first timestamp.
Thus, the desired output would be

1999-APR-8 17:31:06     1500     3     45     0
1999-APR-8 17:31:15     1500     3     45     9
1999-APR-8 17:31:25     1500     3     45     19
1999-APR-8 17:31:30     1500     3     45     24
1999-APR-8 17:31:55     1500     3     45     49
1999-APR-8 17:32:06     1500     3     45     60

How can I deal with this?
Thanks in advance.

2

Try this awk script:

{ split($2, t, ":")
  ts = t[1]*3600+t[2]*60+t[3]
  if (NR == 1) ots = ts
  print $0, ts - ots
}