import datetime

date_string = "Wed, 18 Dec 2013 09:30:00 GMT"
date = datetime.datetime.strptime(date_string, "%a, %d %b %Y %H:%M:%S %Z")
end_date_string = "Wed, 18 Dec 2013 10:15:00 GMT"
end_date = datetime.datetime.strptime(end_date_string, "%a, %d %b %Y %H:%M:%S %Z")
print (end_date-date).seconds/60, "Minutes"
# Difference can get through substracting two datetime objects directly,
 if you have input string then substract after converting to datetime object.
According to your input format change format in while strptime()

The following will work however i suggest you use the answer in your other question because the field that stores the diff can be added to a form with the float time widget to do what i think you are looking for.

time_diff = fields.Datetime.from_string(self.later_datetime) - fields.Datetime.from_string(self.earlier_datetime)
hours_diff = math.floor(round(float(time_diff.days) * 24 + (float(time_diff.seconds) / 3600)))
minute_diff = math.floor((time_diff-hours_diff)*60)