Python trouble converting milliseconds to datetime and back

add: http://stackoverflow.com/questions/29853390/python-trouble-converting-milliseconds-to-datetime-and-back
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from datetime import datetime, timedelta
def datetime_from_millis(millis, epoch=datetime(1970, 1, 1)):
    """Return UTC time that corresponds to milliseconds since Epoch."""
    return epoch + timedelta(milliseconds=millis)

from datetime import datetime, timedelta

def timestamp_millis(utc_time, epoch=datetime(1970, 1, 1)):
    """Return milliseconds since Epoch as integer."""
    td = utc_time - epoch
    return (td.microseconds + (td.seconds + td.days * 86400) * 10**6) // 10**3

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   from datetime import datetime, timedelta
146     def datetime_from_millis(self, millis, epoch=datetime(1970, 1, 1)):
147         """Return UTC time that corresponds to milliseconds since Epoch."""
149         secs = int(millis)/1000
150         return time.strftime("%Y-%m-%d %H:%M:%S",time.localtime(secs))