As it’s the beginning of a new year, I thought it might be interesting to show my spin on creating a custom calendar in Power Query. This topic has been covered by many others, but I’ve never put my own signature on it.

# Our goal

If you’re building calendar intelligence in Power Pivot for custom calendars, you pretty much need to use Rob Collie’s GFTIW pattern as shown below:

=CALCULATE([*Measure*],

ALL(**Calendar445**),

FILTER(

ALL(**Calendar445**),

**Calendar445**[**PeriodID**]=VALUES(**Calendar445**[*PeriodID*])-1

)

)

*Note: The pattern as written above assumes that your calendar table is called “Calendar445”. If it isn’t, you’ll need to change that part.*

This pattern is pretty robust, and, as shown above, will allow you to return the value of the measure for the prior period you provide. But the big question here is how you create the needed columns to do that. So this article will focus on building a calendar with the proper ID columns that you can use to create a 445, 454, 455 or 13 month/year calendar. By doing so, we open up our ability to use Rob Collie’s GFITW pattern for a custom calendar intelligence in Power Pivot.

For more on this pattern see http://ppvt.pro/GFITW

# A bit of background

If you’ve never used one of these calendars, the main concept is this: Comparing this month vs last month doesn’t provide an apples to apples comparison for many businesses. This is because months don’t have an consistent number of days. In addition, comparing May 1 to May 1 is only good if your business isn’t influenced by the day of the week. Picture retail for a second. Wouldn’t it make more sense to compare Monday to Monday? Or the first Tuesday of this month vs the first Tuesday of last month? That’s hard to do with a standard 12 month calendar.

So this is the reason for the custom calendar. It basically breaks up the year into chunks of weeks, with four usual variants:

- 445: These calendars have 4 quarters per year, 3 “months” per quarter, with 4 weeks, 4 weeks and 5 weeks respectively.
- 454: Similar to the 445, but works in a 4 week, 5 week, 4 week pattern.
- 544: Again, similar to 445, but works in a 5 week, 4 week, 4 week pattern
- 13 periods: These calendars have 13 “months” per year, each made up of 4 weeks

The one commonality here is that, unlike a standard calendar, the custom calendar will always have 364 days per year (52 weeks x 7 days), meaning that their year end is different every year.

# Creating a custom calendar

In order to work with Rob’s pattern, we need 5 columns:

- A contiguous date column (to link to our Fact table)
- YearID
- QuarterID
- MonthID
- WeekID
- DayID

With each of those, we can pretty much move backwards or forwards in time using the GFITW pattern.

## Creating a custom calendar – Creating a contiguous date column

To create our contiguous date column, we have a few options. We could follow the steps in this blog post on creating a dynamic calendar table. Or we could skip the fnGetParameter function, and just directly query our parameter table. Whichever method you choose, there is one REALLY important thing you need to do:

**Your calendar start date must be the first date of (one of) your fiscal year(s).**

It can be this year or last year, but you need to determine that. I’m going to assume for this year that my year will start on Sunday, Jan 3, 2016, so I’ll set up a basic table in Excel to hold the dates for my calendar:

Notice the headers are “Parameter” and “Value”, and I also named this table “Parameters” via the Table Tools –> Design tab. For reference, the Start Date is hard coded to Jan 3, 2016, and the End Date is a formula of B4+364*2 (running the calendar out two years plus a day.)

Now I’m ready to pull this into Power Query and build my contiguous list of dates.

- Select any cell in the table –> Create a new query –> From Table
- Remove the Changed Type step (as we don’t really need it)

This should leave you with a single step in your query (Source), and a look at your data table.

- Click the fx button on the formula bar to add a new custom step

This will create a new step that points to the previous step, showing =Source in the formula bar. Let’s drill in to one of the values on the table. Modify the formula to:

=Source**[Value]{0}**

Reading this, we’ve taken the Source step, drilled into the [Value] column, and extracted the value in position 0. (Remembering that Power Query starts counting from 0.)

Now this is cool, but I’m going to want to use this in a list, and to get a range of values in a list, I need this as a number. So let’s modify this again.

=**Number.From(**Source[Value]{0}**)**

Great stuff, we’ve not got the date serial number for our start date. Let’s just rename this step of the query so we can recognize it.

- Right click the Custom1 step –> Rename –> StartDate

Now, let’s get the end date.

- Copy everything in the formula bar
- Click the fx button to create a new step
- Select everything in the formula bar –> paste the formula you copied
- Update the formula as follows:

=Number.From(Source[Value]{**1**})

That should give you the date serial number for the End Date:

Let’s rename this step as well:

- Right click the Custom1 step –> Rename –> EndDate

We’ve now got what we need to create our calendar:

- Click the fx button to create a new step
- Replace the text in the formula bar with this:

={StartDate..EndDate}

If you did this right, you’ve got a nice list of numbers (if you didn’t, check the spelling, as Power Query is case sensitive). Let’s convert this list into something useable:

- Go to List Tools –> Transform –> To Table –> OK
- Right click Column1 –> Rename –> DateKey
- Right click DateKey –> Change Type –> Date
- Change the query name to Calendar445
- Right click the Change Type step –> Rename –> DateKey

The result is a nice contiguous table of dates that runs from the first day of the fiscal year through the last date provided:

## Creating a custom calendar – Adding the PeriodID columns

Now that we have a list of dates, we need to add our PeriodID columns which will allow the GFITW to function.

## Creating a custom calendar – DayID column

This column is very useful when calculating other columns, but can also be used in the GFITW formula to navigate back and forward over days that overlap a year end. To create it:

- Go to Add Column –> Index –> From 1
- Change the formula that shows up in the formula bar to:

=Table.AddIndexColumn(DateKey, "**DayID**", 1, 1)

- Right click the Added Index step –> Rename –> DayID

NOTE: The last two steps are optional. Instead of changing the formula in the formula bar, you could right click and rename the Index column to DayID. Personally, I like to have less steps in my window though, and by renaming those steps I can see exactly where each column was created when I’m reviewing it later.

What we have now is a number that starts at 1 and goes up for each row in the table. If you scroll down the table, you’ll see that this value increases to 729 for the last row of the table. (Day 1 + 364*2 = Day 729).

## Creating a custom calendar – YearID column

Next, let’s create a field that will let us navigate over different years. To do this, we will write a formula that targets the DayID column:

- Go to Add Column –> Add Custom Column
- Name: YearID
- Formula: =Number.RoundDown(([DayID]-1)/364)+1

- Right click the Added Custom step –> Rename –> YearID

If you scroll down the table, you’ll see that our first year shows a YearID of 1, and when we hit day 365 it changes:

The reason this works for us is this: We can divide the DayID by 364 and round it down. This gives us 0 for the first year values, hence the +1 at the end. The challenge, however, is that this only works up to the last day of the year, since dividing 364 by 364 equals 1. For that reason, we subtract 1 from the DayID column before dividing it by 364. The great thing here is that this is a pattern that we can exploit for some other fields…

## Creating a custom calendar – QuarterID column

This formula is very similar to the YearID column:

- Go to Add Column –> Add Custom Column
- Name: QuarterID
- Formula: =Number.RoundDown(([DayID]-1)/91)+1

- Right click the Added Custom step –> Rename –> QuarterID

The result is a column that increased its value every 91 days:

It’s also worth noting here that this value does not reset at the year end, but rather keeps incrementing every 90 days.

## Creating a custom calendar – MonthID column

The formula for this column is the tricky one, and depends on which version of the calendar you are using. We’re still going to create a new custom column, and we’ll call it MonthID. But you’ll need to pick the appropriate formula from this list based on the calendar you’re using:

**Calendar Type** |
**Formula** |

445 |
Number.RoundDown([DayID]/91)*3+
( if Number.Mod([DayID],91)=0 then 0
else if Number.Mod([DayID],91)<=28 then 1
else if Number.Mod([DayID],91)<=56 then 2
else 3
) |

454 |
Number.RoundDown([DayID]/91)*3+
( if Number.Mod([DayID],91)=0 then 0
else if Number.Mod([DayID],91)<=28 then 1
else if Number.Mod([DayID],91)<=63 then 2
else 3
) |

544 |
Number.RoundDown([DayID]/91)*3+
( if Number.Mod([DayID],91)=0 then 0
else if Number.Mod([DayID],91)<=35 then 1
else if Number.Mod([DayID],91)<=63 then 2
else 3
) |

13 periods |
Number.RoundDown(([DayID]-1)/28)+1 |

As I’m building a 445 calendar here, I’m going to go with the 445 pattern, which will correctly calculate an ever increasing month ID based on a pattern of 4 weeks, 4 weeks, then 5 weeks. (Or 28 days + 28 days + 35 days.)

This formula is a bit tricky, and – like the GFITW pattern – you honestly don’t have to understand it to make use of it. In this case this is especially true, as the formula above never changes.

If you’re interested however, the most important part to understand is what is happening in each of the Number.Mod functions. That is the section that is influencing how many weeks are in each period. The key values you see there:

- 0: Means that you hit the last day of the quarter
- 28: This is 4 weeks x 7 days
- 35: This is 5 weeks x 7 days
- 56: This is 8 weeks x 7 days
- 63: This is 9 weeks x 7 days

The Number.RoundDown portion divides the number of days in the DayID column by 91, then rounds down. That will return results of 0 through 3 for any given value. We then multiply that number by 3 in order to return values of 0, 3, 6, 9 (which turns out to be the month of the end of the prior quarter.)

The final piece of this equation is to add the appropriate value to the previous step in order to get it in the right quarter. For this we look at the Mod (remainder) of days after removing all multiples of 91. In the case of the 445, if the value is <= 28 that means we’re in the first 4 weeks, so we add one. If it’s >28 but <=56, that means it’s in the second 4 weeks, so we add two. We can assume that anything else should add 3… except if there was no remainder. In that case we don’t add anything as it’s already correct.

## Creating a custom calendar – WeekID column

WeekID is fortunately much easier, returning to the same pattern we used for the YearID column:

- Go to Add Column –> Add Custom Column
- Name: WeekID
- Formula: =Number.RoundDown(([DayID]-1)/7)+1

- Right click the Added Custom step –> Rename –> WeekID

The result is a column that increases its value every 7 days:

# Finalizing the custom calendar

The last thing we should do before we load our calendar is define our data types. Even though they all look like numbers here, the reality is that many are actually defined as the “any” data type. This is frustrating, as you’d think a Number.Mod function would return a number and not need subsequent conversion.

- Right click the DateKey column –> Change Type –> Date
- Right click each ID column –> Change Type –> Decimal Number
- Go to Home –> Close & Load To…
- Choose Only Create Connection
- Check Add to Data Model
- Click OK

And after a quick sort in the data model, you can see that the numbers have continued to grow right through the last date:

# Final Thoughts

We now have everything we need in order to use the GFITW pattern and get custom calendar intelligence from Power Pivot. Simply update the PeriodID with the period you wish to use. For example, if we had a Sales$ measure defined, we can get last month’s sales using the following:

=CALCULATE([*Sales$*],

ALL(**Calendar445**),

FILTER(

ALL(**Calendar445**),

**Calendar445**[**MonthID**]=VALUES(**Calendar445**[*MonthID*])-1

)

)

As an added bonus, as we’re using Power Query, the calendar will update every time we refresh the data in the workbook. We never really have to worry about updating it, as we can use a dynamic formula to drive the start and end dates of the calendar.

As you can see from reading the post, the tricky part is really about grabbing the right formula for the MonthID. The rest are simple and consistent, it’s just that one that gets a bit wonky, as the number of weeks can change. (To be fair, this would be a problem for the quarter in a 13 period calendar as well… one of those quarter will need 4 weeks where the rest will need 3.)

One thing we don’t have here is any fields to use as Dimensions (Row or Column labels, Filters, or for Slicers.) The reason I elected not to include those here is that the post is already very long, and they’re not necessary to the mechanics of the GFITW formula.

If you’d like a copy of the completed calendar, you can download it here. Be warned though, that I created this in Excel 2016. It should work nicely with Excel 2013 and higher, but you may have to rebuilt it in a new workbook if you’re using Excel 2010 due to the version difference on the Power Pivot Data Model.