Convert timestamp since epoch to datetime.datetime

This is the simplest method I've ever seen-

$ python
Python 2.7.5 (default, Nov  6 2016, 00:28:07) 
[GCC 4.8.5 20150623 (Red Hat 4.8.5-11)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import time
>>> print(time.strftime('%Y-%m-%dT%H:%M:%S %Z',time.localtime(time.time())))
2018-05-02T13:21:44 IST

I would use the time module

>>> import time
>>> time.gmtime(1346114717972/1000.)
time.struct_time(tm_year=2012, tm_mon=8, tm_mday=28, tm_hour=0, tm_min=45, tm_sec=17, tm_wday=1, tm_yday=241, tm_isdst=0)  

This shows the timestamp in UTC/GMT time.

The timestamp is divided by 1000 as the stamps you have provided are in milliseconds since the epoch, not seconds.

Then use strftime to format like so:

>>> time.strftime('%m/%d/%Y %H:%M:%S',  time.gmtime(1346114717972/1000.))
'08/28/2012 00:45:17'

Assuming millisecond resolution:

import datetime

s = '1346114717972'
fmt = "%Y-%m-%d %H:%M:%S"

# local time
t = datetime.datetime.fromtimestamp(float(s)/1000.)
print t.strftime(fmt) # prints 2012-08-28 02:45:17

# utc time
t_utc = datetime.datetime.utcfromtimestamp(float(s)/1000.)
print t_utc.strftime(fmt) # prints 2012-08-28 00:45:17

Have a look at the documentation for the strftime() and strptime() behavior.