Be careful about local time and UTC time when converting Date time to Unix time

In the last post, I paste my python script about converting from local time to UTC time and vice versa.
The reason is I made a mistake when convert Date time to Unix time(Not local time to UTC or UTC to local time).  I treated local time as UTC time and got the wrong results.

Here is the comparison of the correct way and the wrong way:
You can see the below python code, input is "2013-08-23 10:00:00", '%Y-%m-%d %H:%M:%S' which is the same for two converters, but the result in Unix time is DIFFERENT!

>>> calendar.timegm((datetime.datetime.strptime("2013-08-23 10:00:00", '%Y-%m-%d %H:%M:%S')).utctimetuple())
1377252000
>>> int(str(time.mktime(datetime.datetime.strptime("2013-08-23 10:00:00", '%Y-%m-%d %H:%M:%S').utctimetuple()) )[:-2])
1377277200

Why?

The reason is calendar.timegm(t) takes a UTC time as input, and time.mktime(t) takes a local time as input! So simple methods, but it took me way way too long time to find out what exactly happens! 

Here's the python script:

import sys
import datetime
import time
import calendar

#date format 
TIME_FORMAT = '%Y-%m-%d %H:%M:%S'
#hours offset from UTC 
TIME_OFFSET_TO_UTC = 0


#Converts date time to Unix time
def date_to_unix(date_time):
 delta = datetime.timedelta(0,0,0,0,0,TIME_OFFSET_TO_UTC,0)
 return calendar.timegm((datetime.datetime.strptime(date_time, TIME_FORMAT) - delta).utctimetuple())

date_to_unix("2014-02-28 16:24:00")