Ok. Adding the time was easy polulating a datetime field linked to a server action using a python expression

datetime.datetime.now()

Now, how to translate this script

# Credits for the original script to Ken Cochrane: 
# http://stackoverflow.com/questions/2788871/python-date-difference-in-minutes/6879077#6879077 

from datetime import datetime
import time
fmt = '%m/%d/%Y %H:%M:%S'
starttime = '05/27/2015 16:43:38' #pos.order.order_date here
endtime = '05/27/2015 16:47:41' #pos.order.my_custom_field here
d1 = datetime.strptime(starttime, fmt)
d2 = datetime.strptime(endtime, fmt)
# convert to unix timestamp
d1_ts = time.mktime(d1.timetuple())
d2_ts = time.mktime(d2.timetuple())
# they are now in seconds, subtract and then divide by 60 to get minutes.
print int(d2_ts-d1_ts) / 60

to calculate the diff (in minutes) between date_order and my custom field on pos.order on Odoo?

Edit: 

This is the script I put on my model:

def on_change_kot_time(self,cr,user,ids,date_order,x_kot_time2,context=None):
	fmt = '%m/%d/%Y %H:%M:%S'
	d1 = datetime.strptime(date_order, fmt)
	d2 = datetime.strptime(x_kot_time2, fmt)
	d1_ts = time.mktime(d1.timetuple())
	d2_ts = time.mktime(d2.timetuple())
	tempo = int(d2_ts-d1_ts) / 60

	res = {
            'value': {
		'x_minutes': tempo,
	      }    
	}
	return res

I've edit the fields date_order,x_kot_time2 and x_minutes on my xml wiew with the on_change call

 on_change="on_change_kot_time(date_order,x_kot_time2)     

But the fields are not updated. I tried with different type of fields (char, float, datetime) but I'm still stuck.