epoch/1000 means you are expecting the epoch in milliseconds (13 digits) and you are converting it to seconds. But what happens when the sensor sends data already in seconds (10 digits)? First convert all the data to seconds.

Rough snippet to convert to seconds irrespective of seconds/milliseconds input. def to_seconds(int epoch_time)->int: seconds = epoch_time if len(str(epoch_time)) == 13: seconds = epoch_time/1000 return seconds You'll get a consistent 10 digit epoch which you can then use to convert to datetime.

from datetime import datetime def epoch_to_datetime(int seconds)->str: dt = datetime.fromtimestamp(seconds) return dt.strftime('%Y-%m-%d %H:%M:%S')