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:
Any help is much appreciated.
For the special date of January 1, 1970 there are multiple options.
For any other starting date you need to get the difference between the two dates in seconds. Subtracting two dates gives a
timedelta object, which as of Python 2.7 has a
>>> (t-datetime.datetime(1970,1,1)).total_seconds() 1256083200.0
The starting date is usually specified in UTC, so for proper results the
datetime you feed into this formula should be in UTC as well. If your
datetime isn’t in UTC already, you’ll need to convert it before you use it, or attach a
tzinfo class that has the proper offset.
As noted in the comments, if you have a
tzinfo attached to your
datetime then you’ll need one on the starting date as well or the subtraction will fail; for the example above I would add
tzinfo=pytz.utc if using Python 2 or
tzinfo=timezone.utc if using Python 3.
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
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.
int (t.strftime("%s")) also works
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.
from the python docs:
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)
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()
Follow the links if you need solutions for
<3.3 Python versions.