import pytz
import datetime

def string_to_datetime( s ):
    if s is None:
        return None
    try:
        return datetime.datetime.strptime( s, "%Y-%m-%dT%H:%M:%S.%fZ" ).replace( tzinfo = pytc.utc )
    except ValueError:
        return datetime.datetime.strptime( s, "%Y-%m-%dT%H:%M:%SZ" ).replace( tzinfo = pytc.utc )

def datetime_to_string( dt ):
    if dt is None:
        return None
    return dt.astimezone( pytz.utc ).strftime( "%Y-%m-%dT%H:%M:%S.%fZ" )