Converting date and time string to appropriate format for arithmetic calculations

Converting date and time string to appropriate format for arithmetic
calculations

I am using Python and I have dates in the table of the type '2013-06-24
17:04:28' stored as strings and I want to do things such as subtract and
average them.
I dont care much about the time.
What is the appropriate format to convert the string to and how do I do it?
Thanks in advance for any help.
Max.