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Magical DST Formula, or just funky math?

SquidLips — Sun, 23 Nov 2008 20:21:48 -0800

Please help explain why this formula works. So, I had a need to find a formula that would calculate whether or not a particular date was in DST or not. I actually found one, and it works perfect for what I need. But purely put of curiosity, I want to know why it works. Here is the basic formula:

floor(1 + (year * 5 / 4)) % 7

To find the day of March that DST begins, subtract the result from 14. Similarly, to find the date in November when it ends, subtract the result from 7. The formula only works for years 2007 and beyond, when the dates for DST were changed in the US.

Some basic thoughts / assumptions:

Guessing that the formulas are identical (save for the cardinal number to subtract from) because in any given year, March and November always start on the same day of the week.

Prior to 2007, the formulas are different based on which date you are finding, and though both employ roughly similar concepts, the equation is relatively more complex for April; Oct is very similar to the above.

The 14 probably comes from DST starting on the second Sunday of March, and the 7 probably comes from DST ending on the first Sunday in November. The modulo probably comes from 7 days in the week.

The formula works regardless of whether the year is a leap year, but only before the year 2100, which is one of the 'exception to the rule' years which is not a leap year. For years 2100-2199, you have to eliminate the "1 +" part of the equation. Subsequent centuries require similar minor adjustment.

Is the basic formula that simple because of some mathematical coincidence regarding those particular dates, or days of the year? How does it not care about leap years? I would love to have the logic and math explained; understanding this would make some of the other oddball calculations I have to do much easier, I think.

For those wondering... the 'floor' function is a simple function that essentially says 'round down'. So, floor(4.99) would result in 4. And the '%' operator is modulo, which is more or less integer remainder. So 7 % 3 = 1, 14 % 5 = 4, etc.

Thanks in advance to anyone who can help figure this out. Google was no help... heck, I had to weed through dozens of results just to find this formula, much less an explanation.

save me from daylight savings!

slograffiti — Sat, 25 Oct 2008 13:49:34 -0800

HELP! When does Daylight Savings Time end in NYC this year?! ok, so all of my calendars say DST ends tomorrow (10/26/08). However, everywhere I look online etc. says 11/02/08. What is the deal? Did it change? I'm confused, help.

How Does the Military, Police, Etc. Notate the "second" 1:30 am when we "fall back"?

WCityMike — Tue, 06 Nov 2007 08:25:20 -0800

How do places where precise timekeeping is essential handle the repeat of an hour when we go off Daylight Savings Time? There are certain professions in which I imagine very specific denotation of time is particularly important — hospital, military, first-responders (police, fire, ambulance), specialized equipment, etc.

"Spring forward" is probably not a problem, because it "removes" an hour — 2:00 jumps to 3:00. But in the fall, an hour is repeated — that's what I'm curious about.

In these venues, how do they handle properly distinguishing the first 1:30 am from the second 1:30 am? Is it done informally (i.e. Joe the ambulance driver writes down "1:30 am (second time 'round)" in his log while Frank the ambulance driver writes down "1:30 am post-DST" and George the ambulance driver forgets and just writes down "1:30 am"? Or is there a generalized standard in the field.

How to Eliminate Daylight Savings Time?

Jeff Howard — Fri, 31 Aug 2007 12:46:17 -0800

I'd like to build a clock that makes shifting to daylight savings time unnecessary by instead gaining or losing a bit of time each day as appropriate for the season so that when the second Sunday in March and first Sunday in November roll around it's already in sync. Setting aside the logistics of making this work from a societial point of view, how would you do it technically? Something like leap seconds or leap minutes? Or smaller changes on a more constant interval? Now, consider the societal impact. We like to think of time as this holy construct, despite the fact that global synchronized time is a modern invention and Daylight Savings Time is even newer. Since the US government recently changed the rules for when Daylight Savings Time is in force and several communities apparently survive without observing it, the time seems ripe for making the argument that it's possible to do without a semi-yearly shift.

Many people get their time through their cellphone instead of wristwatches, and computer clocks are often synchronized to a remote source, so it seems like it would be possible to distribute post-industrial time to a wide population by getting a few carriers on board. Plus, there's historical precedent for clocks losing time. Many people grew up winding a clock each day, or adjusting the weights in their Grandfather clock. It seems like the shifts would be subtle enough that people could simply adjust any disconnected clocks as necessary every week or so.

I'd like you to help me come up with some mathematical strategies for how to build this clock, then explain to me why it'll never work.

What time is it in Indiana?

jasondigitized — Mon, 04 Jun 2007 12:31:26 -0800

I would like to write a javascript function that takes the current time and adjusts it based on the timezone of any of our customers around the world. "Easy enough if I know the timezone" I thought......until I started to think about daylight savings time and all of its variations around the world. What is the best way to tackle this? Is there a handy script out there to do this, or should I be looking to call a webservice? I still don't know what time it is in Indiana.

PC didn't change for Daylight Savings Time. Now what?

copystar — Sun, 11 Mar 2007 04:20:36 -0800

Uh-oh. My computer (MS XP) didn't make the leap and change itself for daylight savings time. I thought I was ready - I installed all the recommended updates. (Someone left the computer on overnight - that couldn't have caused our trouble, could it?) Is there any I should do other than manually change the clock in the systems tray?

How does my computer know about DST?

Joleta — Thu, 18 Jan 2007 13:33:14 -0800

What (if anything) do I have to do to my computer to make sure it knows about the new U.S. Daylight Savings Time rules? Usually my computer adjusts itself for DST, but how will it know that the dates have changed? I'm running Windows XP Media Center Edition, and I have automatic updates turned on. Will I need to do anything to make sure that the new Daylight Saving Time rules (2nd Sunday in March, 1st Sunday in November) are used? Also, how about my Dell Axim X50v Pocket PC, running Windows Mobile 2003? My husband's Mac (running OSX)?

How prepared is your organization for the Daylight Savings Time changes?

mendel — Wed, 17 Jan 2007 08:24:33 -0800

Mostly directed at IT types: What have you/your department/your company done to prepare for the upcoming changes to the start/end dates for Daylight Savings Time in Canada and the US? I'm really surprised at how little mainstream or IT press the change has been getting so I'm wondering how ready other IT departments are for the change.

(Background: The US Energy Policy Act of 2005 changes the start date of DST from the first Sunday in April to the second Sunday in March, and the end date from the last sunday in October to the first sunday in November, so any software that deals with timezones needs to be updated to account for the change. Most Canadian provinces are implementing the same change.)