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python datetime calculation not trimming to milliseconds

Doing the following calculation to take 30 days of the current date using the date-time module. The calculation is correct but it's not trimming milliseconds to 3 digits. any idea how I can implement it?
from datetime import datetime,timedelta
datetime_limit = datetime.today().strftime('%Y-%m-%d %H:%M:%S.%f')[:-3]
datetime_limit = datetime.today() - timedelta(days=30)

The last digits are added when you do the calculation with timedelta.
from datetime import datetime,timedelta
datetime_limit = str(datetime.today() - timedelta(days=30))[:-3]
print(datetime_limit)

Extract only time (HH:MM) from datetime field - Odoo 10

I'a m trying to extract only time (Hour:Minute) from datetime field
Example:
today_with_hour = fields.Datetime(
string=u'hora',
default=fields.Datetime.now,
)
I would like to know how get only hour from today_with_hour in format
17:10:20

This is one way to extract:
from datetime import datetime
now = datetime.now()
print(str(now.hour)+':'+str(now.minute)+':'+str(now.second))

This may be better way to do it
You can use strftime
Example:
from datetime import datetime
datetime.now().strftime("%H:%M:%S")
In your case you can follow like this:
from datetime import datetime
datetime.strptime('20/06/2019 17:28:52', "%d/%m/%Y %H:%M:%S").time()
Output will be:
17:28:52

Better way to do is by using strftime().
dt = datetime.strptime('20/06/2019 17:28:52', "%d/%m/%Y %H:%M:%S")
dt.strftime("%H:%M:%S")
output:
17:28:52
To get the current time from the datetime.now()
datetime.datetime.now().time().strftime("%H:%M:%S")
O/P:
'11:16:17'
if you want to get the time with milliseconds also, use isoformat()
datetime.datetime.now().time().isoformat()
O/P:
'11:20:34.978272'

Convert local time to UNIX

I'm new to python and I'm trying to get the actual minutes passed every day since 7:00.
I am using mktime to get now_date1 and now_date2 in seconds, and then the plan it's to subtract and divide by 60 to get the minutes.
But I get the following error:
AttributeError: 'str' object has no attribute 'timetuple'
It's this the correct approach?
Here it's the code
import time
import pytz
from datetime import datetime
from time import mktime as mktime
now_date = datetime.now(pytz.timezone('Europe/Bucharest'))
now_date1 = now_date.strftime('%H:%M:%S')
now_date2 = now_date.strftime('7:00:00')
# Convert to Unix timestamp
d1_ts = time.mktime(now_date1.timetuple())

strftime returns a string. Not what you want.
You were pretty close, but there's no need to put time in the mix. Just modify your code like this and use time delta from datetime (inspired by How to calculate the time interval between two time strings):
import pytz
from datetime import datetime
now_date = datetime.now(pytz.timezone('Europe/Bucharest'))
from datetime import datetime
FMT = '%H:%M:%S'
now_date1 = now_date.strftime(FMT)
now_date2 = now_date.strftime('7:00:00')
tdelta = datetime.strptime(now_date1, FMT) - datetime.strptime(now_date2, FMT)
print(tdelta)
I get: 6:40:42 which seems to match since it's 12:42 here.
To get the result in minutes just do:
tdelta.seconds//60
(note that the dates have only correct hour/time/seconds, the year, month, etc.. are 1900 ... since they're not used)

I think something like this might work:
import time
import datetime
from time import mktime as mktime
#current time
now_date = datetime.datetime.now()
#time at 7am
today = datetime.date.today()
now_date2 = datetime.datetime(today.year, today.month, today.day, 7, 0, 0, 0)
#difference in minutes
(now_date - now_date2).days * 24 * 60

Converting between time formats

Is there a way to convert time from the year_month_day-hh_mm_ss to timestapm (in milliseconds since 1971) with DateUtils? or some other library..
thanks.

Have a look at the Python datetime and time modules.
from datetime import datetime
d = datetime.strptime("2017_03_16-14:08:10", "%Y_%m_%d-%H:%M:%S")
This will create a datetime object of d
Then use mktime from Python's time module to get your timestamp
import time
time.mktime(d.timetuple())*1000
The *1000 is required to convert from seconds to milliseconds.
Also, do you mean 1971 or the Unix epoch (Jan 01 1970)?

Try the arrow module found at the following URL: https://pypi.python.org/pypi/arrow

You can parse the time with strptime, then you can get the time since epoch time in milliseconds by using strftime to format only seconds. Multiply by 1000 to get milliseconds.
converted_time.strftime("%s") * 1000

You can use timedelta
from datetime import timedelta
year = timedelta(days=(2017-1971)*365)#number of days from 1971 to 2017
mili_sec = (year.total_seconds())*1000#you will get total_seconds just mulitply with 1000 to get milliseconds
OUTPUT
1450656000000.0
OR
You wanted difference from a particular date.Ex from 1971-01-01 to 2017-03-16-14:08:10
from datetime import datetime
new_day = datetime.strptime("2017_03_16-14:08:10", "%Y_%m_%d-%H:%M:%S")
old_day = datetime.strptime("1971_01_01-00:00:00", "%Y_%m_%d-%H:%M:%S")
diff_day_milliseconds = ((new_day - old_day).total_seconds())*1000
OUTPUT
1458137290000.0

datetime - how to add seconds to Epoch?

I do not even know how to tackle this. I need to first turn a str() into a datetime object, convert it to epoch time add a number of seconds then turn it back into the date in a properly formatted object. A sample of the str is:
"2016-11-04T03:02:00Z"
I'm guessing some regex to break up the str()??

Use a timedelta object, e.g:
from datetime import datetime, timedelta
TIMESTRING_FORMAT = '%Y-%m-%dT%H:%M:%SZ'
dt = datetime.strptime('2016-11-04T03:02:00Z', TIMESTRING_FORMAT)
ndt = dt + timedelta(seconds=5)
print datetime.strftime(ndt, TIMESTRING_FORMAT)
See docs https://docs.python.org/2/library/datetime.html and make sure that my TIMESTRING_FORMAT string is correct.