To get a date string with milliseconds, use [:-3] to trim the last three digits of %f (microseconds):

>>> from datetime import datetime
>>> datetime.utcnow().strftime('%Y-%m-%d %H:%M:%S.%f')[:-3]
'2022-09-24 10:18:32.926'

Or shorter:

>>> from datetime import datetime
>>> datetime.utcnow().strftime('%F %T.%f')[:-3]
'2022-09-24 10:18:32.926'

See the Python docs for more "%" format codes and the strftime(3) man page for the full list.

To convert unix timestamp to datetime, you can use datetime.fromtimestamp(). The only problem, that your timestamp is in miliseconds, but function expect timestamp in seconds. To cut miliseconds you can divide timestamp to 1000.

Code:

from datetime import datetime

a = 1557975599999
date = datetime.fromtimestamp(a // 1000)
print(date)

Output:

2019-05-16 05:59:59

Upd.

@Daniel in comments noticed that fromtimestamp() accept floats, so we can save miliseconds from original timestamp. All we need is just to remove one symbol :D

date = datetime.fromtimestamp(a / 1000)

You want .replace(microsecond=0) to zero out the milli-(and micro-)seconds. Then just specify the right timespec for isoformat, 'milliseconds':

nowzeroed = datetime.now(timezone.utc).replace(microsecond=0)
nowberlin = nowzeroed.astimezone(pytz.timezone("Europe/Berlin"))
nowformatted = nowberlin.isoformat(timespec='milliseconds') 

In Python 3 this can be done in 2 steps:

1. Convert timestring to datetime object
2. Multiply the timestamp of the datetime object by 1000 to convert it to milliseconds.

For example like this:

from datetime import datetime

dt_obj = datetime.strptime('20.12.2016 09:38:42,76',
                           '%d.%m.%Y %H:%M:%S,%f')
millisec = dt_obj.timestamp() * 1000

print(millisec)

Output:

1482223122760.0

strptime accepts your timestring and a format string as input. The timestring (first argument) specifies what you actually want to convert to a datetime object. The format string (second argument) specifies the actual format of the string that you have passed.

Here is the explanation of the format specifiers from the official documentation:

• %d - Day of the month as a zero-padded decimal number.
• %m - Month as a zero-padded decimal number.
• %Y - Year with century as a decimal number
• %H - Hour (24-hour clock) as a zero-padded decimal number.
• %M - Minute as a zero-padded decimal number.
• %S - Second as a zero-padded decimal number.
• %f - Microsecond as a decimal number, zero-padded to 6 digits.