python converting string in localtime to UTC epoch timestamp

问题:

I have strings in YMD hms format that had the timezone stripped. But I know they are in Eastern time with daylight savings time.

I am trying to convert them into epoch timestamps for UTC time.

I wrote the following function:

def ymdhms_timezone_dst_to_epoch(input_str,  tz="US/Eastern"):     print(input_str)     dt = datetime.datetime.fromtimestamp(time.mktime(time.strptime(input_str,'%Y-%m-%d %H:%M:%S')))     local_dt = pytz.timezone(tz).localize(dt)     print(local_dt.strftime('%Y-%m-%d %H:%M:%S %Z%z'))     utc_dt = local_dt.astimezone(pytz.utc)     print(utc_dt.strftime('%Y-%m-%d %H:%M:%S %Z%z'))         e = int(utc_dt.strftime("%s"))     print(e)     return e  Given string `2015-04-20 21:12:07` this prints:      2015-04-20 21:12:07     2015-04-20 21:12:07 EDT-0400 #

which looks ok up to the epoch timestamp. But http://www.epochconverter.com/epoch/timezones.php?epoch=1429596727 says it should mao to Greenwich Mean Time Apr 21 2015 06:12:07 UTC.

What is wrong?

回答1:

I have strings in YMD hms format that had the timezone stripped. But I know they are in Eastern time with daylight savings time.

A portable way is to use pytz:

#!/usr/bin/env python from datetime import datetime import pytz # $ pip install pytz  naive_dt = datetime.strptime('2015-04-20 21:12:07', '%Y-%m-%d %H:%M:%S') tz = pytz.timezone('US/Eastern') eastern_dt = tz.normalize(tz.localize(naive_dt)) print(eastern_dt) # -> 2015-04-20 21:12:07-04:00 

I am trying to convert them into epoch timestamps for UTC time.

timestamp = (eastern_dt - datetime(1970, 1, 1, tzinfo=pytz.utc)).total_seconds() # -> 1429578727.0 

See Converting datetime.date to UTC timestamp in Python.

---

There are multiple issues in your code:

• time.mktime() may return a wrong result for ambiguous input time (50% chance) e.g., during "fall back" DST transition in the Fall

• time.mktime() and datetime.fromtimestamp() may fail for past/future dates if they have no access to a historical timezone database on a system (notably, Windows)

• localize(dt) may return a wrong result for ambiguous or non-existent time i.e., during DST transitions. If you know that the time corresponds to the summer time then use is_dst=True. tz.normalize() is necessary here, to adjust possible non-existing times in the input

• utc_dt.strftime("%s") is not portable and it does not respect tzinfo object. It interprets input as a local time i.e., it returns a wrong result unless your local timezone is UTC.

---

Can I just always set is_dst=True?

You can, if you don't mind getting imprecise results for ambiguous or non-existent times e.g., there is DST transition in the Fall in America/New_York time zone:

>>> from datetime import datetime >>> import pytz # $ pip install pytz >>> tz = pytz.timezone('America/New_York') >>> ambiguous_time = datetime(2015, 11, 1, 1, 30) >>> time_fmt = '%Y-%m-%d %H:%M:%S%z (%Z)' >>> tz.localize(ambiguous_time).strftime(time_fmt) '2015-11-01 01:30:00-0500 (EST)' >>> tz.localize(ambiguous_time, is_dst=False).strftime(time_fmt) # same '2015-11-01 01:30:00-0500 (EST)' >>> tz.localize(ambiguous_time, is_dst=True).strftime(time_fmt) # different '2015-11-01 01:30:00-0400 (EDT)' >>> tz.localize(ambiguous_time, is_dst=None).strftime(time_fmt)  Traceback (most recent call last): ... pytz.exceptions.AmbiguousTimeError: 2015-11-01 01:30:00 

The clocks are turned back at 2a.m. on the first Sunday in November:

is_dst disambiguation flag may have three values:

• False -- default, assume the winter time
• True -- assume the summer time
• None -- raise an exception for ambiguous/non-existent times.

is_dst value is ignored for existing unique local times.

Here's a plot from PEP 0495 -- Local Time Disambiguation that illustrates the DST transition:

The local time repeats itself twice in the fold (summer time -- before the fold, winter time -- after).

To be able to disambiguate the local time automatically, you need some additional info e.g., if you read a series of local times then it may help if you know that they are sorted: Parsing of Ordered Timestamps in Local Time (to UTC) While Observing Daylight Saving Time.

回答2:

First of all '%s' is not supported on all platforms , its actually working for you because your platform C library’s strftime() function (that is called by Python) supports it. This function is what is causing the issue most probably, I am guessing its not timezone aware , hence when taking difference from epoch time it is using your local timezone, which is most probably EST(?)

Instead of relying on '%s' , which only works in few platforms (linux, I believe) , you should manually subtract the datetime you got from epoch (1970/1/1 00:00:00) to get the actual seconds since epoch . Example -

e = (utc_dt - datetime.datetime(1970,1,1,0,0,0,tzinfo=pytz.utc)).total_seconds() 

Demo -

>>> (utc_dt - datetime.datetime(1970,1,1,0,0,0,tzinfo=pytz.utc)).total_seconds() 1429578727.0 

This correctly corresponds to the date-time you get.

回答3:

I don't exactly know why but you have to remove the timezone info from your utc_dt before using %s to print it.

e = int(utc_dt.replace(tzinfo=None).strftime("%s")) print(e) return e 

文章来源: python converting string in localtime to UTC epoch timestamp