> In these situations, the information displayed on a local clock (or stored in a Python datetime instance) is insufficient to identify a particular moment in time.
Does the datetime instance not store the timezone?
That said, I don't see the reason for this PEP. You should only store a value in the DB that doesn't change like that, and then when displayed you can change it to fit the local time of the user.
But once again this is one of those "assumptions about time that everyone gets wrong" just because we shift timezones in Britian doesn't necessarily mean that people in other countries do the same - they might just change what the timezone offset is and keep the timezone the same.
(And related, see https://www.mjt.me.uk/posts/falsehoods-programmers-believe-a... for more about addresses.)
From Larry M. Smith (2006-04-26) re Wisconsin: http://www.legis.state.wi.us/statutes/Stat0175.pdf ... is currently enforced at the 01:00 time of change. Because the local "bar time" in the state corresponds to 02:00, a number of citations are issued for the "sale of class 'B' alcohol after prohibited hours" within the deviated hour of this change every year....
From Winston Churchill (1934-04-28): It is one of the paradoxes of history that we should owe the boon of summer time, which gives every year to the people of this country between 160 and 170 hours more daylight leisure, to a war which plunged Europe into darkness for four years, and shook the foundations of civilization throughout the world.
Well, for a proposed change I'd like to see examples of the actual problem caused. The only example I can see is EST and EDT, which seems to be the same as the UK doing GMT->BST. Same for Andorra.
My question wasn't rhetorical, I was actually asking if there were places that don't do this.
What localities do you know of that just change the hour and do not have a time zone assigned to deal with it? I'm fairly sure the point of time zones it to track all those changes, and a name would be assigned to deal with it.
The important part here is that the API to get a timezone usually doesn't include a Daylight Saving flag. For example pytz would be used as pytz.timezone('US/Central'). There needs to be a process that adjusts the tzinfo object to the proper offset; in pytz this is the localize method, but that's non-standard.
The only time that one has the time fold is when you turn the clock backward (let's just call that shifting from daylight savings to standard timezone). And this would only affect code which used wall clock time (time as it's read, e.g. 1:30am PDT), and would also only affect code which wanted to run something only once at a time within that fold (e.g. 1:30am ... not on both 1:30am's).
So, using the timezone method, just check to see if your current 1:30am is in your daylight savings timezone. Hurray! You're in the clear. Go ahead and do that thing you wanted to do only on the first 1:30am.
But the next day you're going to run into a problem. The only 1:30am you're going to get is in the standard timezone. So now you have to check for this timezone change only on the day of the change, which is yet another piece of data you have to keep track of. On the day of the change, do this timezone comparison, and on every other day don't worry about it.
When the clock hits your interesting time of 1:30am, just check to see if today is the day of the change, check what the current timezone is, check what the daylight savings time zone is, check to see if those those two values are the same, and now do your thing. Otherwise, just do your thing.
All of the above also ignores that people change times at different times (11pm, 1am, 2am, 3am), some don't change a full hour, and some don't change at all.
The proposal gets rid of all of that convoluted logic in everyone's programs, and instead it provides a single boolean value: is this the second time I've seen this time because of daylight savings shenanigans.
Why are you doing time math in local time?
Simply do all the time math in universal time and be done with it.
Does it? It doesn't cover the scheduling problem the other half of the year when the clocks move the other direction.
> So now you have to check for this timezone change only on the day of the change, which is yet another piece of data you have to keep track of.
If running a job twice is a problem, then why not check that the job has not already been run?
> is this the second time I've seen this time
Is this unambiguous? If it's 2015-10-25-01-30-00 GMT, have I seen that time before? In the UK, yes, in Mali no.