# Time patterns

This post introduces the four time-related calculations patterns presented in this website. The goal here is to help you choose the right pattern based on your specific needs. Indeed, when it comes to time-related calculations, the choice of the pattern is hard.

First, what is a time-related calculation? A time-related calculation refers to any calculation that involves time. Examples include the set of period-to-date calculations, like year-to-date, quarter-to-date, or month-to-date. These calculations accumulate values from the beginning of a time period – year, quarter, month – and they return the aggregation of the measure from the start of the period to the date shown in the report. The definition of a time period changes depending on whether you work with the Gregorian calendar or a fiscal calendar. In Figure 1, you can see an example of period-to-date calculations, where YTD stands for year-to-date, and QTD for quarter-to-date.

Included in these patterns are also comparisons of a parameter over a certain period of time, with a different period of time. For example, you can compare the sales of the current month against the sales of the same month in the previous year. Another example of time-related calculations is the moving average over a time period, like a rolling average over 12 months which smoothes out line charts and removes the effect of seasonality from calculations. The four time-related patterns implement the same set of calculations.

What makes the patterns so different from one another, is the definition of what a calendar is. You can already appreciate the different definitions of a year-to-date calculation by looking at Figure 1. Depending on whether you are working with the Gregorian or the fiscal calendar, the numbers are different. When talking about a calendar, things can easily become very complicated because of the definition of the calendar.

For example, you might have a week-based calendar following an ISO standard or your own definition. In a week-based calendar every month starts the same day of the week, and the same goes for the year. Therefore, a year in a week-based calendar might start in the Gregorian year before, or end in the next one. Moreover, some calendars split a year into 13 periods instead of 12 months, for accounting purposes. The calendar requirements are the main driver for the choice of the time-related pattern.

The four time-related patterns are presented in order of increasing complexity:

- Standard time-related calculations
- Month-related calculations
- Week-related calculations
- Custom time-related calculations

# Standard time-related calculations

The **Standard time-related calculations** pattern is implemented using regular DAX time intelligence functions. It works based on the assumption that your calendar is a regular Gregorian calendar and that your fiscal calendar starts at the beginning of a Gregorian quarter. For example, DAX time intelligence functions work fine if your fiscal calendar starts on July 1 (start of the third quarter of a Gregorian calendar). Yet, they might provide unexpected results if your fiscal calendar starts on March 1 – both because March does not start a Gregorian quarter, and because of a historical bug in handling leap years with fiscal calendars. Despite these limitations, the pattern is easy to use and implement because it relies on standard DAX functions and works with a regular date table, with few requirements.

The next three patterns do not use DAX time intelligence calculations. Rather, they are written using basic DAX functions – which leaves much more flexibility in the definition of what a calendar is in terms of quarters, months, and weeks. These patterns require you to build a *Date* table whose columns are required for the DAX measures to identify the fractions of the year. For example, you need one column containing the year, one for the quarter, one for the month, plus additional columns to simplify the calculations.

Moreover, many details need to be considered when detecting and filtering periods. Many calculations that look easy to humans prove to be very complicated for a computer. When you compare one quarter against the previous one, you need to select a different number of days for the two quarters: the January-March quarter is shorter than the April-June quarter. The same goes for the months: January is longer than February, but if you want to make a comparison month-over-month, you need two date selections of different lengths.

If standard time intelligence functions do not meet your needs, then you need to implement one of the other three patterns. All of them require the creation of your own *Date* table.

# Month-related calculations

The **Month-related calculations** pattern is the easiest one. It implements all the calculations assuming that you are not interested in the daily details. For example, if you need calculations and reports that compare one month against another, then the pattern is a good fit. The pattern does not support sub-month selections. If you want to compare three days in a quarter against the same three days in the previous quarter, then you exceed the potential of the pattern: it just does not work. Despite strong limitations in its analytical power (limited to monthly granularity) the month-related pattern is extremely fast and simple to implement. Moreover, it can handle scenarios where you have more than 12 months seamlessly. It comes with the flexibility of a custom-made pattern, and it is simpler than the standard time-related pattern. If you are ok with its limitation about the month granularity, this should be the pattern of choice.

# Week-related calculations

In the **Week-related calculations** pattern, the week is the foundation of the calendar. The ISO 8601 is one of the standards that provide a definition of a week date system – even though many countries adopt different national standards to identify years, quarters, and weeks. One year has 52 or 53 weeks, one quarter has 13 weeks, and each quarter is subdivided into 5+4+4 weeks, 4+5+4 weeks, or 4+4+5 weeks. When there are 53 weeks in a year, there are 14 weeks in one of the quarters. Because a week is not necessarily entirely included in a month, the group of weeks within a quarter should be called a “period” even though it is often referred to as a month. For this reason, we refer to the month names as “periods” in the following description.

Because weeks are the main entity, there is no correspondence between a year in a Gregorian calendar and a year in a week-based calendar. A week-based calendar always starts on the same weekday, like Monday or Sunday. Therefore, only occasionally does this day happen to fall on January 1. For a weekly year, it is totally fine to start on December 29 of the previous year, or on January 3 of the current year. Despite being somewhat unusual, weekly calendars come with some great characteristics: every “month” in a quarter includes the same number of weekdays. Comparing one quarter against another means comparing the same number of days and the same distribution of weekdays.

Week-based calendars require a dedicated *Date* table with several columns to drive the DAX calculations. Moreover, there are no pre-existing DAX functions available to compute calculations over such calendars. Therefore, week-based calculations are implemented with custom DAX code. The complexity is higher than the month-related pattern because the week-related pattern lets you filter any time period, down to the day level. If you have a calendar based on weeks, the week-based calculations pattern is what you have to implement.

# Custom time-related calculations

The **Custom time-related calculations** pattern is the most flexible (and complex) one. This last pattern provides the same calculations as the standard time-related pattern. The relevant difference is that the entire pattern is written using basic DAX functions: we do not use any DAX time intelligence functions. Consequently, the pattern is extremely flexible because you can freely change the behavior of the calculations. With greater flexibility comes greater complexity.

The DAX code of the last pattern is not trivial. It requires much attention to small details. Use it only if none of the other patterns satisfies your business requirements, and you really need the complete flexibility it provides.

Finally, which pattern should you choose?

- If your requirements are satisfied by a regular Gregorian calendar, the
**Standard time-related calculations**pattern is the obvious choice. - If the month granularity is enough for your reporting needs – which is often the case, more often than expected – then the
**Month-related calculations**pattern is the optimal choice: fast and simple. - If you work with a calendar based on weeks, then you need the
**Week-related calculations** - If none of the above is enough and you really need total flexibility, be prepared for a long and fascinating trip into the intricacies of filter contexts and dive straight into the
**Custom time-related calculations**

Remember: with a Business Intelligence project, simpler is better. Choose the most straightforward pattern that satisfies your needs. Needless to say, if you are curious about the differences between the various implementations, it might be useful to have a quick read through all four patterns before making your choice.

This pattern is designed for Power BI / Excel 2016-2019. An alternative version for Excel 2010-2013 is also available.

This pattern is included in the book DAX Patterns, Second Edition.