pytz and Etc/GMT-5

Question

I'm having trouble understanding the conversion between the "Etc/GMT-5" timezone and UTC in pytz.

>>> dt = datetime(2009, 9, 9, 10, 0) # September 9 2009, 10:00
>>> gmt_5 = pytz.timezone("Etc/GMT-5")
>>> gmt_5.localize(dt)
datetime.datetime(2009, 9, 9, 10, 0, tzinfo=<StaticTzInfo 'Etc/GMT-5'>)

Everything is fine so far, but then I try to convert that to UTC:

>>> gmt_5.localize(dt).astimezone(pytz.utc)
datetime.datetime(2009, 9, 9, 5, 0, tzinfo=<UTC>)

So to me it seems that when converting from 10:00 in GMT-5 to UTC I get 05:00? I would expect pytz to give me 15:00 instead.

What am I missing?

EDIT: I have confirmed that timezone conversion for the US/Eastern timezone works just as I'd expect:

>>> eastern = pytz.timezone("US/Eastern")
>>> eastern.localize(dt)
datetime.datetime(2009, 9, 9, 10, 0, tzinfo=...) # Too long
>>> pytz.utc.normalize(eastern.localize(dt).astimezone(pytz.utc))
datetime.datetime(2009, 9, 9, 14, 0, tzinfo=<UTC>)

EDIT 2: I have confirmed that when I use Etc/GMT+5 I get 15:00, which is what I'd expect to get from Etc/GMT-5. Is this a pytz bug?

Answer 1

This is apparently a POSIX thing. From Wikipedia:

In order to conform with the POSIX style, those zones beginning with "Etc/GMT" have their sign reversed from what most people expect. In this style, zones west of GMT have a positive sign and those east have a negative sign.