http://qiita.com/shibainurou/items/0b0f8b0233c45fc163cd

from datetime import datetime as dt

tstr = '2012-12-29 13:49:37'
tdatetime = dt.strptime(tstr, '%Y-%m-%d %H:%M:%S')


strptimeの第二引数は第一引数のフォーマットを渡す。
 例えば、
tstr = '2012/12/29 13:49:37'だった場合、
dt.strptime(tstr, '%Y/%m/%d %H:%M:%S')


文字列から日付(date) 

import datetime

tstr = '2012-12-29 13:49:37'
tdatetime = datetime.datetime.strptime(tstr, '%Y-%m-%d %H:%M:%S')
tdate = datetime.date(tdatetime.year, tdatetime.month, tdatetime.day)


日付から文字列 

from datetime import datetime as dt

tdatetime = dt.now()
tstr = tdatetime.strftime('%Y/%m/%d')