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Daylight Savings Time Changes in Power Query

In this post, I’ll show how to accommodate for Daylight Savings Time changes in a Power BI file. The video I’ve included will show the formulas that I use and how they adapted from Daylight Savings Time to Standard Time. There are many ways to do this and it is best to find a way that accommodates what you need to do, but I’ll walk through how I do it for my needs within Power Query.

  • The first thing I do in Power Query is to identify the start and end date of Daylight Savings Time. In the US, the start date is the second Sunday in March and end date was the first Sunday in November and I’ve added the time of 2:00am.
  • You could look these up and make a list of them for the next 8 or 10 years but to do it formulaically, I use a DateTime.LocalNow and Date.Year to pull the year out of whatever year we are in.
  • In the Applied Steps area in my query settings, I create a small temporary calendar table using that the year that will start in March and end in November. I create a list of those dates under source and then I convert that list into the actual dates.
  • No I have a calendar table with column1 being all dates listed from the start of March thru November. Using the Added Custom fields in Applied Steps, I add a column for month number and one for number of the day of the week (0-6).
  • Since I’m only interested in Sundays in March and November, I filter this table to show me only Sundays (day of week 0) in those months. More specifically, I need to know the second Sunday in March and the first Sunday in November. To do that, I want to group rows by their month number and then add an index column.
  • If I click on one of the columns in my table, I can see a Sunday index which is counting in order (1st, 2nd, 3rd Sunday). If I remove the other columns and just keep the one with the index and expand it, I see I have all the information pieces I need (month number, Sunday index columns).
  • Next, I’ll add a little helper column (GetDays) that will combine the month number and Sunday index columns together. I filter that helper column for the 2nd Sunday in March and the 1st Sunday in November.
  • So, I have all I need but I just need to add a time, so I’ll put a time component in and combine it with the date, so I have a DateTime column.
  • It’s important to note that when we do comparisons in Power Query between dates, date times or date time zone, you want to be consistent with your format. If I’m going to compare date time zones, I need to put my date times into a date time zone. I also need to ensure that the time zone is the same, so I’m going to use a consistent switch. I’ll add a time zone to my start and end dates.
  • I’ve added two queries (StartTime and EndTime) in which I’ll reference that DateTime list. In my formulas, I’ll write in the DateTime in March as the min and the end DateTime in November as the max. I’ll also need to add the time zone to each and make sure I’m consistent. Check out my video for more detail of the formulas.
  • The logic here is an If/Then statement. If the switch time zone which I’ve set to -4 (which is Eastern Time during Daylight Savings), is greater than the start time and less than the end time, then we are in Daylight Savings and the time is correct.
  • If it’s not in between those two dates, then we’re not in Daylight Savings and we need to subtract an hour from it because now we’re going to fall back. In other words, we’re going to be 5 hours behind instead of four. If I refresh it again, it should match the time that’s on my computer. In my SwitchTime Zone column, I see the time that it was in the summer and in my DateTime with DST Correction column I see the correct time now which matches my computer, so I know that this is working correctly.
  • But how do I know it’s working to the exact second? I’ll just duplicate my three queries and create a testing query. In this testing query I will arbitrarily set the start and end time to be a specific day and time to check back on my table to see if it matches. This testing helps ensure that it’s working perfectly.

My video below goes into more detail and may make this written demo clearer. I hope my peek into how I can accommodate for Daylight Savings within Power Query is helpful. This is certainly not the only way to do this, but it works well for me, so I thought it was worth sharing.

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Steve WiseDaylight Savings Time Changes in Power Query