Temporal Table Time Zones

Louis Davidson talks about how to translate temporal table start and end dates to your local time zone:

In my first 5 blogs on Temporal, I failed to note something pretty important about their usage. The start and end times for the row (and it follows, the historical rows) are likely not in your local time zone. They are stored in UTC time zone. This is obviously good because of that pesky daylight saving time issue where 1:30 AM occurs twice each year (here in the US, the time occurs on the first Sunday of November).

Unless you live in London, England or someplace where the offset from UTC is 0, it can make working with these tables confusing, because most people rarely work in UTC time, and even rarer is to think in UTC time when most of your data is likely in your local time zone. So you write your query and use local time…and then, most likely, data is returned…but not necessarily the data you actually desired.

Click through to see ways of translating those values.

Adding Public Holidays To A Date Dimension

Reza Rad continues his series on Power BI date dimensions:

To get public holidays live, you first need an API that is giving you up-to-date information. There are some web pages that has the list of public holidays. I have already explained in another blog post how to use a web page and query public holidays from there. That method uses custom functions as well, here you can read about that.

The method of reading data from a web page has an issue already; Web.Page function from Power Query is used to pull data from that page, and this function needs a gateway configuration to work. There is another function Xml.Document that can work even without the gateway. So because of this reason, we’ll use Xml.Document and get data from an API that provides the result set as XML.

WebCal.fi is a great free website with calendars for 36 countries which I do recommend for this example. This website, provides the calendars through XML format. There are other websites that give you the calendar details through a paid subscription. However, this website is a great free one which can be used for this example. WebCal.fi is created by User Point Inc.

This was an interesting approach to the problem, one I did not expect when first reading the article.  I figured it’d be some sort of date calculation script.

Working With AT TIME ZONE In SQL Server

Louis Davidson has a post up showing how to use AT TIME ZONE:

I have a series of blogs about temporal tables in SQL Server 2016, which you can see here. However, one thing I never had mentioned was that the Temporal Row Start and End Times are always stored in UTC times (which is generally inconvenient to work with unless you are from the Great Britain area).  However, when you are looking at your data and want to see what the data looked like at ‘Yesterday at 12:00 at the computer you are working from, this can be a bit more work.

In 2016 SQL Server, a new feature was added that totally passed me by. AT TIME ZONE. It is used to manipulate the time zone offset of a point in time value,either to append the time zone to a value, or to shift the time zone value of a DATETIMEOFFSET value. The most common practical use that I will show is converting a value in your local time to UTC (and vice versa).

This is something I tend to forget about, but can be useful when building quick reports from UTC.  I’d store all data in UTC, just to make sure dates and times are consistent across servers in different locations, but you don’t have to do those calculations in your head all the time.

Working With UTC And Local Times

Jo Douglass shows how to use the DATETIMEOFFSET data type and AT TIME ZONE syntax to convert between UTC and local times:

Run select SysDateTimeOffset(); and you should see a date and time which mirrors your server’s current time, plus a time zone offset showing its current offset from UTC; this includes any time zone offset, plus any daylight savings time offset.

If I were to run this (from the UK) on August 15th, 2017 while my clock is showing that it’s noon exactly, I would get 2017-08-15 12:00:00.0000000 +01:00; the +01:00 offset is because the UK is offset by one hour from UTC during daylight savings. The datetime2 portion of a datetimeoffset is in local time, not UTC.

My normal operation is to store everything in UTC and let the application convert to local times.  That allows you to compare dates much more easily and reduces confusion around daylight savings time.

Annoying Date Formats

Randolph West shows that even The Best Date Format can deceive you under certain circumstances:

Look carefully. DATE and DATETIME2 are showing the date of 12 July 2017 as expected. Unfortunately, the DATETIME and SMALLDATETIME data types are showing a date of 7 December 2017.

That’s not good at all. It means that the ISO 8601 standard does not work the way we might expect it to. The reason is simple, if annoying: we need to add a time to the date to make it pass the ISO 8601 standard.

I don’t like the idea of having to write 20170713 instead of 2017-07-13, but that is the only date format in SQL Server that I’ve run across that will work with any language and culture settings.

STOP Date Formats

Dave Mason notes that the STOPAT date option when restoring a log backup is temperamental:

There’s nothing I see in the documentation regarding the format for “time“. But there are a couple of examples, including this one:

RESTORE LOG AdventureWorks
FROM AdventureWorksBackups
WITH FILE=4, NORECOVERY, STOPAT = 'Apr 15, 2020 12:00 AM';

That string looks suspiciously like a US English date format. I suspect that wouldn’t work for languages that don’t recognize “Apr” as a month. And what if the date is displayed in one of the many date formats used outside of the US? Lets find out!

Dave tried 21 different date formats; click through for the results.

Avoid Ticks

Michael J. Swart shows you how to convert DATETIME2 values to Ticks:

A .Net tick is a duration of time lasting 0.1 microseconds. When you look at the Tick property of DateTime, you’ll see that it represents the number of ticks since January 1st 0001.
But why 0.1 microseconds? According to stackoverflow user CodesInChaos “ticks are simply the smallest power-of-ten that doesn’t cause an Int64 to overflow when representing the year 9999”.

Even though it’s an interesting idea, just use one of the datetime data types, that’s what they’re there for. I avoid ticks whenever I can.

I agree with Michael:  avoid using Ticks if you can.

Getting A Date Is Hard

Nate Johnson has a fun rant about datetime ranges in SQL Server and date pickers:

I mean, I’m not that old, but spinning thru a few decades is still slower than just typing 4 digits on my keyboard — especially if your input-box is smart enough to flip my keyboard into “numeric only” mode.

Another seemingly popular date-picker UX is the “calendar control”.  Oh gawd.  It’s horrible!  Clicking thru pages and pages of months to find and click (tap?) on an itty bitty day box, only to realize “Oh crap, that was the wrong year… ok let me go back.. click, click, tap..” ad-nauseum.

Food for thought.

JSON Dates In SQL Server

Bert Wagner explains how to handle JSON datetime strings in SQL Server:

In SQL Server, datetime2’s format is defined as follows:

YYYY-MM-DD hh:mm:ss[.fractional seconds]

JSON date time strings are defined like:

YYYY-MM-DDTHH:mm:ss.sssZ

Honestly, they look pretty similar. However, there are few key differences:

  • JSON separates the date and time portion of the string with the letter T

  • The Z is optional and indicates that the datetime is in UTC (if the Z is left off, JavaScript defaults to UTC). You can also specify a different timezone by replacing the Z with a + or  along with HH:mm (ie. -05:00 for Eastern Standard Time)

  • The precision of SQL’s datetime2 goes out to 7 decimal places, in JSON and JavaScript it only goes out to 3 places, so truncation may occur.

Read on for a few scripts handling datetime conversions between these types.

That 53rd Week

Jens Vestergaard notes that you can sometimes have a 53rd week in the year:

There are a lot of great examples out there on how to build your own custom Time Intelligence into Analysis Services (MD). Just have a look at this, this, this, this and this. All good sources for solid Time Intelligence in SSAS.
One thing they have in common though, is that they all make the assumption that there is and will always be 52 weeks in a year. The data set I am currently working with is built on ISO 8601 standard. In short, this means that there is an (re-) occurrence of a 53rd full week as opposed to only 52 in the Gregorian version which is defined by: 1 Gregorian calendar year = 52 weeks + 1 day (2 days in a leap year).

The 53rd occurs approximately every five to six years, though this is not always the case. The last couple of times  we saw 53 weeks in a year was in 1995, 2000, 2006, 2012 and 2015. Next time will be in 2020. This gives you enough time to either forget about the hacks and hard-coded fixes in place to mitigate the issue OR bring your code in a good state, ready for the next time.

Dates and currency are hard problems.

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