[python] In Python, how do you convert a `datetime` object to seconds?

Apologies for the simple question... I'm new to Python... I have searched around and nothing seems to be working.

I have a bunch of datetime objects and I want to calculate the number of seconds since a fixed time in the past for each one (for example since January 1, 1970).

import datetime
t = datetime.datetime(2009, 10, 21, 0, 0)

This seems to be only differentiating between dates that have different days:

t.toordinal()

Any help is much appreciated.

To get the Unix time (seconds since January 1, 1970):

>>> import datetime, time
>>> t = datetime.datetime(2011, 10, 21, 0, 0)
>>> time.mktime(t.timetuple())
1319148000.0

To convert a datetime object that represents time in UTC to POSIX timestamp:

from datetime import timezone

seconds_since_epoch = utc_time.replace(tzinfo=timezone.utc).timestamp()

To convert a datetime object that represents time in the local timezone to POSIX timestamp:

import tzlocal # $ pip install tzlocal

local_timezone = tzlocal.get_localzone()
seconds_since_epoch = local_timezone.localize(local_time, is_dst=None).timestamp()

See How do I convert local time to UTC in Python? If the tz database is available on a given platform; a stdlib-only solution may work.

Follow the links if you need solutions for <3.3 Python versions.

Python provides operation on datetime to compute the difference between two date. In your case that would be:

t - datetime.datetime(1970,1,1)

The value returned is a timedelta object from which you can use the member function total_seconds to get the value in seconds.

(t - datetime.datetime(1970,1,1)).total_seconds()

Maybe off-the-topic: to get UNIX/POSIX time from datetime and convert it back:

>>> import datetime, time
>>> dt = datetime.datetime(2011, 10, 21, 0, 0)
>>> s = time.mktime(dt.timetuple())
>>> s
1319148000.0

# and back
>>> datetime.datetime.fromtimestamp(s)
datetime.datetime(2011, 10, 21, 0, 0)

Note that different timezones have impact on results, e.g. my current TZ/DST returns:

>>>  time.mktime(datetime.datetime(1970, 1, 1, 0, 0).timetuple())
-3600 # -1h

therefore one should consider normalizing to UTC by using UTC versions of the functions.

Note that previous result can be used to calculate UTC offset of your current timezone. In this example this is +1h, i.e. UTC+0100.

References:

• datetime.date.timetuple
• time.mktime
• datetime.datetime.fromtimestamp
• introduction in time module explains POSIX time, 1970 epoch, UTC, TZ, DST ...

The standard way to find the processing time in ms of a block of code in python 3.x is the following:

import datetime

t_start = datetime.datetime.now()

# Here is the python3 code, you want 
# to check the processing time of

t_end = datetime.datetime.now()
print("Time taken : ", (t_end - t_start).total_seconds()*1000, " ms")

Starting from Python 3.3 this becomes super easy with the datetime.timestamp() method. This of course will only be useful if you need the number of seconds from 1970-01-01 UTC.

from datetime import datetime
dt = datetime.today()  # Get timezone naive now
seconds = dt.timestamp()

The return value will be a float representing even fractions of a second. If the datetime is timezone naive (as in the example above), it will be assumed that the datetime object represents the local time, i.e. It will be the number of seconds from current time at your location to 1970-01-01 UTC.

from the python docs:

timedelta.total_seconds()

Return the total number of seconds contained in the duration. Equivalent to

(td.microseconds + (td.seconds + td.days * 24 * 3600) * 10**6) / 10**6

computed with true division enabled.

Note that for very large time intervals (greater than 270 years on most platforms) this method will lose microsecond accuracy.

This functionality is new in version 2.7.

I tried the standard library's calendar.timegm and it works quite well:

# convert a datetime to milliseconds since Epoch
def datetime_to_utc_milliseconds(aDateTime):
    return int(calendar.timegm(aDateTime.timetuple())*1000)

Ref: https://docs.python.org/2/library/calendar.html#calendar.timegm

int (t.strftime("%s")) also works