Does anyone know why python's dateutil reverses the sign of the GMT offset when it parses the datetime field?
Apparently this feature is a known outcome of not only dateutil but also other parsing functions. But this results in an incorrect datetime result unless a pre-processing hack is applied:
from dateutil import parser
jsDT = 'Fri Jan 02 2015 03:04:05.678910 GMT-0800'
python_datetime = parser.parse(jsDT)
print(python_datetime)
>>> 2015-01-02 03:04:05.678910+08:00
jsDT = 'Fri Jan 02 2015 03:04:05.678910 GMT-0800'
if '-' in jsDT:
jsDT = jsDT.replace('-','+')
elif '+' in jsDT:
jsDT = jsDT.replace('+','-')
python_datetime = parser.parse(jsDT)
print(python_datetime)
>>> 2015-01-02 03:04:05.678910-08:00
It seems dateutil
uses POSIX-style signs here. It is not related to Python. Other software does it too. From the tz database:
# We use POSIX-style signs in the Zone names and the output abbreviations,
# even though this is the opposite of what many people expect.
# POSIX has positive signs west of Greenwich, but many people expect
# positive signs east of Greenwich. For example, TZ='Etc/GMT+4' uses
# the abbreviation "GMT+4" and corresponds to 4 hours behind UT
# (i.e. west of Greenwich) even though many people would expect it to
# mean 4 hours ahead of UT (i.e. east of Greenwich).
The tz database is used almost everywhere.
Example:
$ TZ=Etc/GMT-8 date +%z
+0800
You probably expect a different timezone:
>>> from datetime import datetime
>>> import pytz
>>> pytz.timezone('America/Los_Angeles').localize(datetime(2015, 1, 2, 3, 4, 5, 678910), is_dst=None).strftime('%Y-%m-%d %H:%M:%S.%f %Z%z')
'2015-01-02 03:04:05.678910 PST-0800'
Note: PST
, not GMT
.
Though dateutil
uses POSIX-style signs even for the PST
timezone abbreviation:
>>> from dateutil.parser import parse
>>> str(parse('2015-01-02 03:04:05.678910 PST-0800'))
'2015-01-02 03:04:05.678910+08:00'
datetime.strptime()
in Python 3 interprets it "correctly":
$ TZ=America/Los_Angeles python3
...
>>> from datetime import datetime
>>> str(datetime.strptime('2015-01-02 03:04:05.678910 PST-0800', '%Y-%m-%d %H:%M:%S.%f %Z%z'))
'2015-01-02 03:04:05.678910-08:00'
Notice the sign.
Despite the confusion due to POSIX-style signs; dateutil
behavior is unlikely to change. See dateutil
bug: "GMT+1" is parsed as "GMT-1" and @Lennart Regebro's reply:
Parsing GTM+1 this way is actually a part of the Posix specification. This is therefore a feature, and not a bug.
See how TZ
environment variable is defined in the POSIX specification, glibc uses similar definition.
It is not clear why dateutil
uses POSIX TZ
-like syntax to interpret the timezone info in a time string. The syntax is not exactly the same e.g., POSIX syntax requires a semicolon: hh[:mm[:ss]]
in the utc offset that is not present in your input.
The source code for dateutil.parser.parse explains this.
Check for something like GMT+3, or BRST+3. Notice that it doesn't mean "I am 3 hours after GMT", but "my time +3 is GMT". If found, we reverse the logic so that timezone parsing code will get it right.
And a further comment:
With something like GMT+3, the timezone is not GMT.
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