What it does: Returns an Integer containing the specified part of a given date

Syntax: DatePart(interval, date[,firstdayofweek[,firstweekofyear]])

There are two ways to use this function:

• Using the DateInterval collection
• Using a string expression

Using the DateInterval Collection

Let’s start with an example:

To return the current month: =DatePart(DateInterval.Month, Today())

Now, in the expression builder (in BIDS), it will look like there’s something the matter with what we have just written:

You can ignore this, the expression will work. An important point to note when using the DateInterval collection, is that you must use a date that is of type Datetime. If you use a string representation of a date (“2009/11/13”), then you’ll need to convert this to Datetime using CDate(“2009/11/13”) before the expression will work. Thus:

This will work:  =DatePart(DateInterval.Month, CDate(“2009/11/13”))

This won’t:  = DatePart(DateInterval.Month, “2009/11/13”)

When you use the DatePart function to identify the day of the week you can specify what the First Day Of The Week is (defaults to Sunday if this is not specified). You can do this by accessing the FirstDayOfWeek collection. Example – To Return the day of the week, specifying Monday as the first day of the week:

DatePart(DateInterval.Weekday, CDate(“2009/11/13”), FirstDayOfWeek.Monday) returns 5 (Friday)

All the possibilities for using the DatePart() function with the DateInterval Collection:

The following table shows all the possibilities for the DatePart Function using the DateInterval collection. All expressions use a Parameter called MyDate (type = Datetime), which has a value of “2009/11/13 12:34:23”

Using a String Expression

Again, let’s start with an example:

To return the current month:  =DatePart(“m”, Today())

Using a String expression for your interval enables you to use a date that is of datatype String:

=DatePart(“m”, “2009/11/13”)

All the possibilities for using the DatePart() function with a String Expression:

As noted previously, using a string expression for the interval allows you to use a date value that is of the String datatype. You can of course use a Datetime value if you wish to.

This table shows all the possibilities for the DatePart function using a String Expression for the interval. All expressions use a Parameter called MyDate (type = String), which has a value of “2009/11/13 12:34:23”

Is there a preferred method?

Ultimately, no. At the end of the day, all the results are the same. What is my preference? I use the DateInterval collection, mainly becuase I like that fact that you can’t get confused about which date part you are using (w/ww, y/yyyy, n….). It’s very clear to anyone who is reading the code that (for example) DateInterval.WeekOfYear refers to the week of the year and nothing else. It also forces you to use a Datetime Value, which safeguards against invalid dates (sort of).

I hope this clears things up a little!

The DateAdd() function. According to BOL this “Returns a date to which a specified time interval has been added”. The structure of the function is as follows:

DateAdd(interval, number, date)

It’s a Date!

Like most first dates, getting to grips with this function and understanding its ‘needs’ is a tricky process. Initially, you may have attempted to use the tried-and-trusted methods (aka ’syntax’), DateAdd(mm, 1, Now()). No. Ok, let’s try DateAdd(‘mm’, 1, now()). Nope. What about DateAdd(month, 1, Now()). Good luck with that…Yes. It’s a little mystifying.

Score!

There are a couple of ways to achieve the desired result with this function. 1.) use the DateInterval collection, or 2.) use the date part abbreviations. My recommendation? Use the DateInterval collection. Using the abbreviations, there is a chance that something will be calculated differently to what you’d expect (I explain a little further on). The DateInterval collection can be found in the Object Browser and looks like this:

Fig 1. A list of all the Intervals in the DateInterval collection

“So how do you use this in the function?” I hear you ask.

=DateAdd(DateInterval.Month, 1, Today()) which gives you a date 1 month from today.

This will work for all the intervals listed in Fig 1. above.

If you don’t want to use this method and instead want to use the abbreviations then there are a few things to be weary of. For instance, using DateAdd(“w”, 1, Today()) does not yield a date 1 week from today, but rather 1 day from today. This happens with a couple of other abbreviations as well. The reason is that “w” represents “weekday” not “week”. Here is an shortlist of the abbreviations that do work:

• Days ["dd"] – DateAdd(“dd”, 1, today())
• Weeks ["ww"] – DateAdd(“ww”, 1, today())
• Months ["mm"] – DateAdd(“mm”, 1, today())
• Years ["yyyy"] – DateAdd(“yyyy”, 1, today())

For a full list of abbreviations and further explanation, msdn delivers here.

Hopefully this clears up a little of the confusion surrounding the DateAdd() function and the intervals that need to be specified. Of course, if there are other methods that can be used or I’m all ears.

I like keep an open mind. When working with windows, one must.