What do you mean by "number of seconds" ? The number of seconds from when? From what epoch? (without an epoch it is meaningless to count seconds).
The definition of MATLAB's serial date number is clearly defined: "the number of days from January 0, 0000". If you want this time in seconds then simply multiply the serial date number by the number of seconds in a day. Note that these numbers will be much larger!
If you only need the seconds component of those timestamps, then use datevec, and take a look at the sixth column of data:
>> C = {'2013-01-01 00:00:00' '2013-01-01 00:00:01' '2013-01-01 00:00:02'};
>> V = datevec(C)
V =
2013 1 1 0 0 0
2013 1 1 0 0 1
2013 1 1 0 0 2
>> V(:,6)
ans =
0
1
2
