data['Time']=data.TimeStr.apply(lambdax:time.mktime(dt.datetime.strptime(x,'%Y-%m-%dT%H:%M:%S.%fZ').timetuple()))#This is not UTC. It does not really matter.
del(data['TimeStr'])
data.columns=['session_id','timestamp','item_id']
data['Time']=data.timestamp.apply(lambdax:time.mktime(dt.datetime.strptime(x,'%Y-%m-%dT%H:%M:%S.%fZ').timetuple()))#This is not UTC. It does not really matter.