useful info on Daylight Savings Time - clocks move forward this Sunday (March 10, 2024)

[Anil Philip]

Since DST is on the certification exam, wanted to share this article in Reuters that I found useful.

Daylight saving time always starts on the second Sunday in March and ends on the first Sunday in November for the United States

Daylight saving time is the practice of moving clocks forward by one hour during summer months so daylight lasts longer into the evening.

I had not realized the reason behind DST and also when exactly it occurs.

[Peter Rooke]

It can vary, on a previous project we just mandated using UTC.  Of course, we had one system that could not be changed, so all other systems had to be altered twice every year!  

Here's an insight Computerphile - The Problem with Time & Timezones ;-)  

[Paul Anilprem]

>Daylight saving time always starts on the second Sunday in March and ends on the first Sunday in November for the United States
I think it used to start on first Sunday of April and end on last sunday of Octorber ((a decade or so ago?).

But you don't need to remember these dates for the exam. If the answer depends on the when DST starts or ends, the problem statement will tell you when it starts/ends. You do need to know, for example, what happens to a ZonedDateTime object when you add an hour to the ZonedDateTime on this boundary.

[Tim Holloway]

Paul Anilprem wrote:>Daylight saving time always starts on the second Sunday in March and ends on the first Sunday in November for the United States
I think it used to start on first Sunday of April and end on last sunday of Octorber ((a decade or so ago?).

Except when Congress changes it.  And it should be noted that a number of states have voted to make DST permanent and it's just waiting for Congress to approve it. If they can ever spare the time from pointless political stunts.

Of course, DST is also locale-specific. Different rules apply to parts of Indiana, the state of Arizona, the Navaho Nation, and the Hopi Nation within the Navaho Nation, and so forth.

So keep that timezone file/tzdata package up to date!

As a side note, all my servers have their hardware clocks set on UTC, though at the user level what I normally see is local time.

[Anil Philip]

Paul Anilprem wrote:
I think it used to start on first Sunday of April and end on last sunday of Octorber ((a decade or so ago?).

Yes, I remember they changed the dates.

[Anil Philip]

Tim Holloway wrote:And it should be noted that a number of states have voted to make DST permanent

What exactly does that mean - will we have to change clocks twice a year without arguing about it - or will we never change our clocks again?

[Tim Holloway]

A number of states voted this in, but speaking specifically for Florida, the intent is that the entire state should move to DST all year around.

Florida, incidentally, is deliberately off-kilter anyway. Its longitude indicates that it should properly lie in the Central Time Zone, but when timezones were first established, it was common for Damnyankees from Up North to come down for the winter via steamships or railroads and the timezone was established for their convenience (not having to change the times on their clocks and watches).

West of the the Appalachicola River in the Panhandle, they gave up, and put that part of the state in Central Time. Since the tourists of the day were more interested in the St. Johns River, Daytona, and Miami, that was not a problem.

[Anil Philip]

Actually I have a basic question on daylight savings time. Since in the winter it gets dark earlier, why don't they do the opposite and move the clocks forward in the winter time so that we can get more daylight by starting the day earlier?

[Anil Philip]

Anil Philip wrote:Actually I have a basic question on daylight savings time. Since in the winter it gets dark earlier, why don't they do the opposite and move the clocks forward in the winter time so that we can get more daylight by starting the day earlier?

Ignore. I was confused between moving clocks forward and backward.

[Mike Simmons]

Anil Philip wrote:What exactly does that mean - will we have to change clocks twice a year without arguing about it - or will we never change our clocks again?

It means no more changes, you just stay on DST.

Alternately, permanently having no DST also has this same effect.  One of the things I miss about living in Arizona.

Just on general principle, I find it silly to make the one with "Summer" in the norm.  "Standard" should be the norm.

And of course, none of these actually change the amount of sunlight in the day.  They just shift the work/school hours a bit to try to fit better.

[Campbell Ritchie]

Mike Simmons wrote:. . . none of these actually change the amount of sunlight in the day.  They just shift the work/school hours a bit to try to fit better.

Sunlight? I have vague memories of seeing that once.

We had Summer time all winter, well over fifty years ago. It was called standard time. It was a total disaster; it only achieved moving work hours to fit a lot worse. Florida might manage being an hour ahead of itself all twelve months of the year because they have so much day light when everybody else has winter. Come to think of it, northerrn Alaska or the Yukon could do the same, for exactly the opposite reason!

[Ron McLeod]

Yukon switched to permanent daylight savings time 4 years ago.

The Canadian province of BC (where I live), along with Washington and Oregon states are waiting for California to make the switch; afterwards all of north american pacific coast will be using permanent daylight savings time.

[Anil Philip]

Mike Simmons wrote:
It means no more changes, you just stay on DST.

Will there be a missing "hole" in Time?
Currently, in one year, the shifts cancel out. +1 hour in summer - 1 hour in winter = 0
But if we go permanently from 1:59 am to 3 am, I wonder what the side effects are for that 1-hour hole in time.

[Paul Clapham]

Anil Philip wrote:Will there be a missing "hole" in Time?
Currently, in one year, the shifts cancel out. +1 hour in summer - 1 hour in winter = 0
But if we go permanently from 1:59 am to 3 am, I wonder what the side effects are for that 1-hour hole in time.

There's no hole, we're just talking about what we call the time. There would be a gap in that, sure, just like there was a gap in the calendar back in 1582 when Pope Gregory made that happen. The birds in the trees were totally unaffected by that and went about their business as usual, but people were affected because their monthly payment schedules were thrown out of whack. You might think that people would be affected by the permanent switch to (or from) DST if they were paid hourly, but since the jumps forward and backwards have been happening for years, people's payment procedures already know how to adjust for that.

[Paul Clapham]

Being a thing devised by people, Daylight Saving Time is an outrageously complicated beast. Java programmers are fortunate because one of the things Java includes is a database of all of the daylight savings rules everywhere in the world going back as far as possible, so all you really need to know is how DST works. And those rules continue to change -- have a look at this article for examples of recent changes. Or have a look at Daylight saving time in Australia for a description of the rules there.

[Mike Simmons]

I think we all agree that the underlying physics of the universe are unaffected by this, as are all the animals - it's just the humans and the way we represent our dates and times that are affected.  But whether we call it a "hole" or a "gap", yes, there is something weird going on here.  In Spring we jump directly from 1:59:59 AM to 3:00:00 AM.  And in Fall, we go from 1:59:59 AM to... 1:00:00 AM, and then repeat an hour.  Technically we go from 1:59:59 AM DST to 1:00:00 AM Standard Time, if you need to distinguish between the two.

Someone has probably written a murder mystery where the solution depends on the fact that the same clock time occurs twice during fall DST adjustments.  For a select audience, I imagine.

[Campbell Ritchie]

Mike Simmons wrote:. . . a murder mystery . . . the same clock time occurs twice . . . .

That book should only be read in October, which is the longest month (at least here), and that gives you more time to work out the solution.

[Tim Holloway]

I concur that when it's 12:00 noon, the sun should be directly overhead at the meridian, but the business world thinks that you can bend reality by the "power of positive thinking" and make everyone more "productive" by screwing with the clocks. So instead of universal standard time, we get universal daylight time.

It's recorded fact that changes in schedules cause an increase in both medical events and workplace accidents. And one of the most significant problems hereabouts is that schoolchildren are more likely to become accident victims when they find themselves waiting for the bus in the dark.

So we could deal with time based on measured evidence or we could deal with time the way we "know" is right. And of the two, history suggests that the Sun will wander into a black hole before the first option is ever considered for public policy.

[Anil Philip]

Mike Simmons wrote:I think we all agree that the underlying physics of the universe are unaffected by this, as are all the animals - it's just the humans and the way we represent our dates and times that are affected.

I was thinking along the lines of whether there are programs (business, scientific...) that need to step backwards in time.
a) I wondered whether they handle the missed hour and extra hour every year, correctly. Just like most people can walk forwards correctly but walking backwards will cause most to stumble and fall.
b) if and when DST becomes permanent, I wonder whether the bump in history when we last lost an hour, will be taken into account by such programs. I wonder if there is a Y2K-like moment here.

[Anil Philip]

Campbell Ritchie wrote:

Mike Simmons wrote:. . . a murder mystery . . . the same clock time occurs twice . . . .

That book should only be read in October, which is the longest month (at least here), and that gives you more time to work out the solution.

Thanks for the laugh

[Tim Holloway]

Anil Philip wrote:

Mike Simmons wrote:I think we all agree that the underlying physics of the universe are unaffected by this, as are all the animals - it's just the humans and the way we represent our dates and times that are affected.

I was thinking along the lines of whether there are programs (business, scientific...) that need to step backwards in time.
a) I wondered whether they handle the missed hour and extra hour every year, correctly. Just like most people can walk forwards correctly but walking backwards will cause most to stumble and fall.
b) if and when DST becomes permanent, I wonder whether the bump in history when we last lost an hour, will be taken into account by such programs. I wonder if there is a Y2K-like moment here.

It's ironic. Allegedly, DST was invented to benefit farmers, but of all the professions, farming is one of the least governed by clocks and the most governed by natural light. On the other hand, those Dark Satanic Mills were originally lit by high windows, so if you could hurry in the workers when the sun rose early, it was that much more work you could squeeze out of them.

Astronomers use Julian Days, which are measured in seconds and decimal fractions thereof from the JD Epoch, That means that they don't care about Leap Years, Daylight Savings or even leap seconds. The stars don't have any need for such frivolities. Space missions often count days/hours/minutes to/from launch or some other mission milestone. Businesses historically haven't done much clock-sensitive processing in the wee hours of Sunday morning, so they didn't care, although a completely 24x7 real-time world-wide system has its own considerations.

Putting a bump or hole in timekeeping on a permanent basis would have no more impact than the present crop of semi-annual hiccups we must endure.

I'm pretty sure that the standard TZDATA resources that are kept updated in Unix and Windows OS's also maintain histories of the different changeover dates back to the point where a clock and calendar were more governed by personal preference and the Church/State.

Realize that formal timezones only came about in the early 1800s or so, for the benefit of railroads. Before that, you'd set your clock by the local sun. Or sometimes a cannon or flag at an installation which got their time from the local sun. One of the original essential functions of the Greenwich Observatory. In their case, the master time was used to set ships chronometers for the benefit of determining longitude based on the difference between Greenwich and the local sun.

The fun part is that you can calculate backwards, but not forwards. Because you never know what shenanigans may arise. The Leap Second has been permanently abandoned, for example, if I read the news aright.

[Anil Philip]

Tim Holloway wrote:It's ironic. Allegedly, DST was invented to benefit farmers, but of all the professions, farming is one of the least governed by clocks and the most governed by natural light. On the other hand, those Dark Satanic Mills were originally lit by high windows, so if you could hurry in the workers when the sun rose early, it was that much more work you could squeeze out of them.

Astronomers use Julian Days, which are measured in seconds and decimal fractions thereof from the JD Epoch, That means that they don't care about Leap Years, Daylight Savings or even leap seconds. The stars don't have any need for such frivolities. Space missions often count days/hours/minutes to/from launch or some other mission milestone. Businesses historically haven't done much clock-sensitive processing in the wee hours of Sunday morning, so they didn't care, although a completely 24x7 real-time world-wide system has its own considerations.

Putting a bump or hole in timekeeping on a permanent basis would have no more impact than the present crop of semi-annual hiccups we must endure.

I'm pretty sure that the standard TZDATA resources that are kept updated in Unix and Windows OS's also maintain histories of the different changeover dates back to the point where a clock and calendar were more governed by personal preference and the Church/State.

Realize that formal timezones only came about in the early 1800s or so, for the benefit of railroads. Before that, you'd set your clock by the local sun. Or sometimes a cannon or flag at an installation which got their time from the local sun. One of the original essential functions of the Greenwich Observatory. In their case, the master time was used to set ships chronometers for the benefit of determining longitude based on the difference between Greenwich and the local sun.

The fun part is that you can calculate backwards, but not forwards. Because you never know what shenanigans may arise. The Leap Second has been permanently abandoned, for example, if I read the news aright.

Thank you - that was a good read!

[Campbell Ritchie]

Tim Holloway wrote:. . . Allegedly, DST was invented to benefit farmers . . .

Are you sure about that? I remember reading abot William Willett when I was at school, and he didn't particularly seem to be concerned about farmers. Nor does this Wikipedia page support those allegations.

Dark Satanic Mills . . . .

Beware of anything William Blake said; I have heard that he intended, “Dark Satanic Mills,” to mean the Church of England. Not the industrial revolution's factories.

[Stephan van Hulst]

Mike Simmons wrote:Just on general principle, I find it silly to make the one with "Summer" in the norm.  "Standard" should be the norm.

It depends on whether summer time or winter time is on average a better fit for your society.

For instance, in the Netherlands we have the problem that during winter time, we're on UTC+1 in order to fit in with our central European business partners, even though topographically, we should be in the same time zone as Great Britain. Summer time makes it even worse: during DST we're at UTC+2. If we adopted summer time (UTC+2) as our standard year-round, our winters would be terrible because it would still be dark at 9 o'clock in the morning. So for us it would be better to adopt winter time as our standard year-round time.

The inverse would be true to countries that adapted their time zone to business partners to the west of them. For them it would make more sense to adopt summer time as their standard.

[Paul Clapham]

Stephan van Hulst wrote:For instance, in the Netherlands we have the problem that during winter time, we're on UTC+1 in order to fit in with our central European business partners, even though topographically, we should be in the same time zone as Great Britain. Summer time makes it even worse: during DST we're at UTC+2. If we adopted summer time (UTC+2) as our standard year-round, our winters would be terrible because it would still be dark at 9 o'clock in the morning...

Thinking of your time zone relative to UTC makes you think differently about it. For example if you say "Yukon switched to permanent DST" that seems different than "The Yukon is permanently 7 hours behind Coordinated Universal Time, also called UTC -7" (the latter is how the government's web site describes it).

(In mid-December, with that rule the sun rises at 11 am in Whitehorse but it still sets before 5 pm. There's only so much daylight to go around.)

[Stephan van Hulst]

You can't save daylight if there's none to be saved.

[Mike Simmons]

Stephan van Hulst wrote:

Mike Simmons wrote:Just on general principle, I find it silly to make the one with "Summer" in the norm.  "Standard" should be the norm.

It depends on whether summer time or winter time is on average a better fit for your society.

Well, there are various valid (and less valid) reasons to alter the time zone one way or another - and I wasn't objecting to that, in and of itself.  But whatever you call the new time zone, please don't call it "Summer" if it's year-round, because that part sounds like nonsense to me.

Sadly, the world's governments have failed to consult me on this matter, so far.  Oh well.

[Campbell Ritchie]

Stephan van Hulst wrote:. . . topographically, we should be in the same time zone as Great Britain.

Spain is west of Britain and also on CET.

. . . our winters would be terrible because it would still be dark at 9 o'clock in the morning. . . . .

Our winters were terrible for that very reason, in 1968‑1971. And it was dark before we finished work at 5. That is why summer time doesn't make sense in the Winter. It might in Iceland, where they only have a few hours of daylight in midwinter anyway.

[Mike Simmons]

Stephan van Hulst wrote:. . . topographically, we should be in the same time zone as Great Britain.

Topographically the Netherlands has a range of elevations from -7 m to 322.4 m, while Great Britain ranges from -2.75 m to 1345 m.  Exclude Scotland. and the remainder would be more comparable to the Netherlands.  But in general, the main topograpic difference between the two is right there in the name Netherlands.  

Campbell Ritchie wrote:Spain is west of Britain and also on CET.

 Parts of Spain are a little to the west and parts are a little to the east.  But overall it's mostly to the south.

I gather the intended point was that geographically it seems like Spain and Britain should be in the time zone, but they aren't.  I agree - it's an example of countries choosing time zone for other reasons.

[Paul Clapham]

Mike Simmons wrote:Sadly, the world's governments have failed to consult me on this matter, so far.  Oh well.

I think they need your help out here on the west coast. No government wants to change unilaterally to no more ST/DT switches, they (rightly) want all of the jurisdictions to change at the same time. So finally California has stepped up and said they're seriously thinking of changing to permanent standard time, as of November. Change the clocks back and that's it, we're done. And here in BC our premier is saying, like, this is great because in our survey 93% of the people wanted permanent daylight time. I think somebody is missing out on a detail there.

[Mike Simmons]

Stephan van Hulst wrote:It appears I was asleep at the wheel. I meant to say "Geographically".

I know; I just had to needle you for it.  My main objective was to give Campbell crap for his geography, but as long as your posts were right next to each other...

[Santosh K Sahu]

"Standard Time" should be the norm instead of Daylight Saving Time (DST) stems from a preference for natural daylight patterns and consistency. Standard Time reflects the sun's position more accurately, meaning that noon is closer to when the sun is at its highest point. This alignment makes daily routines, such as waking up and commuting, feel more in sync with natural light.

By contrast, DST artificially shifts an hour of daylight from the morning to the evening, which can create problems like darker mornings, sleep disruption, and health risks when clocks change. For many, especially those who rise early or have children going to school, darker mornings during DST are inconvenient and can increase risks related to commuting and safety.

The word "Standard" implies a default or a norm, and some find it illogical to make "Summer Time" or DST the year-round standard. They argue that keeping Standard Time all year would reduce confusion, maintain a better balance of daylight, and avoid the twice-yearly clock shifts that cause inconvenience and health issues. This view aligns with the idea that human routines should match natural light as closely as possible, making Standard Time feel more practical and sustainable.

[Campbell Ritchie]

Santosh K Sahu wrote:"Standard Time" should be the norm . . .

That is a matter of opinion which many will disagree with.

a preference for natural daylight patterns and consistency. . . . daily routines, such as waking up and commuting, feel more in sync with natural light. . . .

Since we ceased to be a principally agricultural society, we gave up synchronising waking and sleeping with the exact times of night and day.
You are right that summer time throughout winter (we called it “British Standard Time”) is a disaster in countries far from the equator where winter nights are long. I well remember it