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Category: Dates and Numbers

Dates in Base R

Published 2019-04-10 by Kevin Feasel

Michael Toth explains some of the functionality available in base R (that is, not packages like lubridate) for working with dates:

When working with R date formats, you’re generally going to be trying to accomplish one of two different but related goals:

1. Converting a character string like “Jan 30 1989” to a Date type

2. Getting an R Date object to print in a specific format for a graph or other output

You may need to handle both of these goals in the same analysis, but it’s best to think of them as two separate exercises. Knowing which goal you are trying to accomplish is important because you will need to use different functions to accomplish each of these. Let’s tackle them one at a time.

There are some good insights in the post. H/T R-bloggers

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The Pain of DST and the Lessened Pain with AT TIME ZONE

Published 2019-03-06 by Kevin Feasel

Bert Wagner shows how you can use AT TIME ZONE as of SQL Server 2016 to make dealing with Daylight Savings Time a little less painful:

The fallacy above is that I said our two datetime2’s are in UTC, but SQL Server doesn’t actually know this. The datetime2 (and datetime) datatype doesn’t allow for time zone offsets so SQL Server really doesn’t know what time zone the data is in.

Using AT TIME ZONE on a datetime2 without offset information causes SQL Server to “…[assume] that [the datetime] is in the target time zone”. That explains why the two datetime2s above, intended to be in UTC, are actually seen as Eastern Daylight Time by SQL Server.

Read the whole thing. Dates and times are a lot more difficult than they first appear. And then they turn out to be a lot more difficult than that.

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Using SWITCHOFFSET

Published 2019-01-31 by Kevin Feasel

Doug Kline has a video and T-SQL script around date/time offsets and particularly the SWITCHOFFSET function:

— so, before SWITCHOFFSET existed, …

SELECT SWITCHOFFSET(SYSDATETIMEOFFSET(),'-05:00') AS [EST the easy way], TODATETIMEOFFSET(DATEADD(HOUR, -5, SYSDATETIMEOFFSET()), '-05:00') AS [EST the hard way]

— so, thinking of a DATETIMEOFFSET data type as a complex object

— with many different parts: year, month, day, hour, time zone, etc.

— it looks like SWITCHOFFSET changes two things: time zone and hour

This was an interesting video. I typically think entirely in UTC and let the calling application convert to time zones as needed, but if that’s not an option for you, knowing about SWITCHOFFSET() is valuable.

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Daylight Savings Time Calculations In Power BI

Published 2019-01-30 by Kevin Feasel

Fred Kaffenberger shows us how to convert UTC to local time zones with daylight savings time:

Quick tip for DST Refresh Date function Power BI Service. I’ll put the code up front, and explain it below. I’ll also say a bit about how to use it at the end. The United States and other places, like Australia, have a pesky thing called Daylight Savings Time. This means that in Central Time US, the offset from Universal Time Coordinated (UTC) is sometimes -6 and other times it’s -5. While Power Query can convert time zones, it doesn’t handle DST. And, my users like to see when the reports were refreshed as a step in evaluating data quality. In 2019, US DST is from March 10 – November 3 (2 AM local time). So, the functions here need to be updated every year.

As promised, here’s the custom function. 

Click through for the custom function and a nice explanation of how it works.

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How Query Store Stores Date Data

Published 2019-01-07 by Kevin Feasel

Erin Stellato shows us a few nuances to the way that Query Store handles dates internally:

Query Store retains query performance data at the plan level.  This data is then broken out into intervals of time, determined by the INTERVAL_LENGTH_MINUTES setting.  The time intervals are found in the sys.query_store_runtime_stats_interval system view, and the start_time and end_time columns are of the DATETIMEOFFSET data type.  This means that the date is time-zone aware, and in Query Store the data is stored as UTC.  Now why does all of this matter?  Because handling dates in Query Store is important if you’re going to query the data directly.

Click through to see a query of what this looks like, as well as a few tips on parsing the data.

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DATEDIFF

Published 2018-11-23 by Kevin Feasel

Randolph West continues a series on covering dates and times, looking at DATEDIFF and DATEDIFF_BIG:

The only functional difference between them is that the DATEDIFF_BIG() returns values as a BIGINT, for results that exceed the boundary of an INT. Keep this in mind when deciding which one to use. For example, the maximum number of seconds an INT can hold is 68 years, while a BIGINT can comfortably store the number of seconds in 10,000 years. This becomes especially important when dealing with microseconds and nanoseconds.

The rest of the post will use DATEDIFF() to refer to both functions.

I think this might be the first time I’d read about DATEDIFF_BIG()and I’m not aware of ever having used it.  But hey, it could make sense if you need to track more than 2 billion microseconds.

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Working With Dates And Times In T-SQL

Published 2018-09-07 by Kevin Feasel

Tomaz Kastrun walks us through various functions to work with dates and times in T-SQL:

Manipulating date and time in T-SQL is a daily and very common task that every DBA, SQL Developer, BI Developer and data scientist will come across. And over the years, I have accumulated many of the simple date or/and time manipulation combinations of different functions, that it is time, to put them together.

Don’t expect to find here anything you haven’t used or seen – especially, if you are a long time T-SQL developer. The point is to have a post, that will have a lot of examples on date and time manipulation on one place. And by no means, this is not the definite list, but should be quite substantial and the code on Github repository will be update.

The list will be updated on my Github, and therefore this blogpost might not include all. In all of the following examples I will be using function GETDATE() to get the current datetime, unless the examples will have stored dates. Therefore, some of the examples or screen-prints will be different from yours.

This mostly focuses on the DATETIME type rather than DATETIME2 or DATE, but there are a few TIME uses.  Check out Tomaz’s repo for more.

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Using AT TIME ZONE In SQL Server

Published 2018-09-06 by Kevin Feasel

Randolph West looks at the AT TIME ZONE clause when working with a specific time zone in SQL Server:

The time zone name is taken from a list maintained in the following Windows registry hive: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time Zones

Note: For SQL Server on Linux (and Docker containers), a registry shim performs the same function as the Windows registry by intercepting the API calls and returning the expected value(s).

We can also use a Transact-SQL (T-SQL) query against the system view sys.time_zone_info, which uses the information from the registry hive. This is the recommended method if you do not have access to the registry hive.

Click through for a couple of examples.

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The Two-Digit Cutoff For Years

Published 2018-09-05 by Kevin Feasel

Claudio Silva explains why a two-digit year may be interpreted differently in SQL Server versus Excel:

What do you read when you see some date in a format like “01-Jan-00 00:00:00.000”? Keep in mind that I’m talking about the output directly from the table and without any formatting.
1st of January seems to leave no doubt (just because there is no default date format starting with two digits for the year), but…what about the year part ’00’?
It stands for 1900 and the 3rd column is wrong?
Or it stands for 2000 and the DATEPART function is returning the wrong value?

This is why you want to stick with four-digit years.  But if you’re stuck with two-digit years for some reason, Claudio explains how you can get Excel and SQL Server to return the same results.

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