Compare launch time of EC2 instance and current time in python

I extract the launch_time from EC2 instance, it returns a unicode string like this:

2014-12-22T08:46:10.000Z

I use dateutil parser to convert it to datetime with

launch_time = parser.parse(instance.launch_time)

so I get lunch_time after converted like this:

2014-12-22 08:46:10+00:00

And I want to compare this launchtime with current time to see how long this instance has been running.

I get current_time with:

current_time = datetime.datetime.now()

and I get it like this:

2014-12-22 11:46:10.527010

Now I have two timestamps, I have this function

def timeDiff(launch_time, current_time):
    running_time = current_time - launch_time
    return running_time.seconds/60

I expect the result would be 180 minutes (3 hours). But I got this error:

TypeError: can't subtract offset-naive and offset-aware datetimes

I think there's obvious difference between these two timestamps. I need to compare exactly date and time to see how long it has been running. I couldn't find a proper way to solve this. Any thoughts appreciated!