Just for the record, this
>> common_date = datetime(14,1,1,0,0,0)
>> actual_dates = '29-Jan-2014 13:57:30'
>> t_diff = actual_dates - common_date
does automatically convert the string (no need to explicitly call datetime on it, although of course it doesn't hurt). It seems a little funny that you are creating a datetime in the year 14 (not 2014), but I'll assume you meant to do that. The result simply says that there are 17.5 million hours between those two dates. This
>> t_diff.Format = 'y'
is probably a more useful format, or, as StarStrider suggested, use the between function to get the difference in calendar units.