Dates and time zone codes around a DST change

I'm testing how dates are calculated and displayed (with time zone codes) around a daylight savings change.

In the UK, at 1am on the 30th March 2014, we enter DST, and go from GMT to BST. The time jumps from 2014-03-30 00:59:59 GMT to 2014-03-30 02:00:00 BST.

I've hit a strange issue replicating this with the following code:

import pytz
from datetime import datetime, time, timedelta

def is_dst(d, tz):
    assert d.tzinfo is None  # we want a naive datetime to localize
    return tz.localize(d).dst() != timedelta(0)

start_datetime = datetime(2014, 03, 30, 0, 0, 0)
tz = pytz.timezone('Europe/London')

# Increment using timedelta
print 'This doesn\'t work:'
d = start_datetime
for i in range(5):
    print str(d) + ' ' + tz.tzname(d, is_dst=is_dst(d, tz))
    d += timedelta(minutes=30)  # Add 30 minutes

# Increment by adding seconds to epoch
print 'This works:'
epoch = datetime.utcfromtimestamp(0)
timestamp = (start_datetime - epoch).total_seconds()
for i in range(5):
    d = datetime.fromtimestamp(timestamp)
    print str(d) + ' ' + tz.tzname(d, is_dst=is_dst(d, tz))
    timestamp += 30 * 60  # Add 30 minutes

The output is:

This doesn't work:
2014-03-30 00:00:00 GMT
2014-03-30 00:30:00 GMT
2014-03-30 01:00:00 GMT <- invalid time
2014-03-30 01:30:00 GMT <- invalid time
2014-03-30 02:00:00 BST
This works:
2014-03-30 00:00:00 GMT
2014-03-30 00:30:00 GMT
2014-03-30 02:00:00 BST
2014-03-30 02:30:00 BST
2014-03-30 03:00:00 BST

I have marked on the output where the invalid times are. Those times do not exist on the wallclock, there is no 1am or 1:30am on the 30th March 2014, so I'm not sure why it is being displayed.

The same process but done in a slightly different way yields the correct results. Why is this?

Solution

Actually, both sections of code are incorrect. Running your exact code on my machine (in US Pacific Time zone), the bottom section returns:

2014-03-29 17:00:00 GMT
2014-03-29 17:30:00 GMT
2014-03-29 18:00:00 GMT
2014-03-29 18:30:00 GMT
2014-03-29 19:00:00 GMT

This is because fromtimestamp uses the computer's local time zone when none is specified. If I just switch the fromtimestamp to utcfromtimestamp, it will use a naive value - The results then get put in the correct time zone, but it gives the same results as the first section - showing the two invalid times.

The problem is rectified by using the normalize method from the pytz timezone instance. Unfortunately, you're keeping d as a naive datetime, so you can't normalize it without localizing it first, and when done you'll have to make it naive again. This works, but makes for some messy code:

# Increment using timedelta
print 'This works:'
d = start_datetime
for i in range(5):
    print str(d) + ' ' + tz.tzname(d, is_dst=is_dst(d, tz))
    d += timedelta(minutes=30)  # Add 30 minutes
    d = tz.normalize(tz.localize(d)).replace(tzinfo=None)

# Increment by adding seconds to epoch
print 'This works too:'
epoch = datetime.utcfromtimestamp(0)
timestamp = (start_datetime - epoch).total_seconds()
for i in range(5):
    d = tz.normalize(datetime.fromtimestamp(timestamp, pytz.utc).astimezone(tz)).replace(tzinfo=None)
    print str(d) + ' ' + tz.tzname(d, is_dst=is_dst(d, tz))
    timestamp += 30 * 60  # Add 30 minutes

Output:

This works:
2014-03-30 00:00:00 GMT
2014-03-30 00:30:00 GMT
2014-03-30 02:00:00 BST
2014-03-30 02:30:00 BST
2014-03-30 03:00:00 BST
This works too:
2014-03-30 00:00:00 GMT
2014-03-30 00:30:00 GMT
2014-03-30 02:00:00 BST
2014-03-30 02:30:00 BST
2014-03-30 03:00:00 BST

Of course, the whole thing could be simplified by using aware datetimes early:

import pytz
from datetime import datetime, time, timedelta

tz = pytz.timezone('Europe/London')
start_datetime = tz.localize(datetime(2014, 03, 30, 0, 0, 0))

# Increment using timedelta
print 'This works:'
d = start_datetime
for i in range(5):
    print str(d) + ' ' + d.tzname()
    d = tz.normalize(d + timedelta(minutes=30))  # Add 30 minutes

# Increment by adding seconds to epoch
print 'This works too:'
epoch = datetime.fromtimestamp(0, pytz.utc)
timestamp = (start_datetime - epoch).total_seconds()
for i in range(5):
    d = datetime.fromtimestamp(timestamp, pytz.utc).astimezone(tz)
    print str(d) + ' ' + d.tzname()
    timestamp += 30 * 60  # Add 30 minutes

Output:

This works:
2014-03-30 00:00:00+00:00 GMT
2014-03-30 00:30:00+00:00 GMT
2014-03-30 02:00:00+01:00 BST
2014-03-30 02:30:00+01:00 BST
2014-03-30 03:00:00+01:00 BST
This works too:
2014-03-30 00:00:00+00:00 GMT
2014-03-30 00:30:00+00:00 GMT
2014-03-30 02:00:00+01:00 BST
2014-03-30 02:30:00+01:00 BST
2014-03-30 03:00:00+01:00 BST

Note that you no longer need the is_dst function, as you can now get the tzname directly from the aware datetime instance.