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Dates…What’s the difference?

Published October 31, 2017 by Javier Menendez

Some days ago (or long, long ago, depending when do you read these lines) I received an email from a developer that was porting code from his old VisualBasic domain to the native, multi-platform Xojo. He asked me how can to get the difference between two dates? I’m pretty sure that most of you will have the answer, but I told him he’ll need Xojo.Core.Date and Xojo.Core.DateInterval. If you want to know how easy it is or how to get the same result for all your code based on the old date class, then I invite you to continue reading…

Use the new Framework, Luke

The Xojo.Core.Date and Xojo.Core.DateInterval classes under the new, Xojo framework make it really easy to get the difference between two given dates. The code you’ll need for this is as easy as this:

Using Xojo.Core
Dim dateA As date = New Date(2017,10,23, TimeZone.current)
Dim dateB As date = New Date(2017,11,23, TimeZone.Current)
Dim result As DateInterval = dateB - dateA
MsgBox result.Years.ToText+ " " + result.Months.ToText + " " + result.Days.ToText

The key here is in the use of the subtract operator that gives as result a TimeInterval instance. Thus, accessing the properties of this instance will get the years, months and days elapsed between the two compared dates. Take into account the importance of the order of your Date operands when used with the subtract operator. I mean, if the rightmost date is higher than the leftmost date, then probably you will get some negative values for the properties. In some cases, this will be the expected behaviour for the logic of your app… other times, you will prefer to check this before getting the DateInterval instance in order to get proper values.

Date difference for existing code

But, what if you want to get the same function for your current code based on the date class? Here you can opt for several options. The one we are going to put in practice is based in the use of the Xojo Module in combination with the Class Extension OOP feature. In fact, Class Extension is the way we can extend the functionallity of an existing Class. Straight, right? So, in order to achieve this the first step, add a new Module to a Xojo project and name it using the Inspector Panel (for example, “Extents”). Then, select the newly created module and add a new Method to it using the following signature:

  • Method Name: Difference
  • Parameters: Extends d As Date, RightDate As Date
  • Return Type: Dictionary

Next, we will add some text constants to the Module, that will be used by the Method and help any calling code that wants to access the result.

  • kDay As String = Day
  • kDay As String = Month
  • kYear As String = Year

Afterwards, select the Difference method and type the following lines of code in the resulting Code Editor:

Dim difference As Double = RightDate.TotalSeconds - d.TotalSeconds
Dim days As Integer
Dim years As Integer
Dim months As Integer
Dim dateResult As New date
dateResult.TotalSeconds = difference
days = dateResult.Day - 1
years = dateResult.Year - 1904
months = dateResult.Month - 1
Dim result As New Dictionary
result.Value(kDay) = days
result.Value(kYear) = years
result.Value(kMonth) = months
Return result

As you can see, the code is simple enough. In order to simplify the code to a maximum, this example opts to give the result as an Instance of Dictionary and using the previously definied constant to store the “Year”, “Months” and “Days” of difference between the compared dates. If you want, you can use an additional Structure or Class to return the result (your own DateInterval?).

To test the new Class Extension, add an Open Event to the Window or App object of your project and type this lines of code:

Dim d As New date(2017,10,24)
Dim d1 As New date(2017,11,30)
Dim interval As Dictionary = d.Difference(d1)
MsgBox interval.Value(kYear) + " " + interval.Value(kMonth) + " " + interval.value(kDay)

A new Date Class that makes the Difference

There is a third option that we can explore that is more OOP oriented: create our own Date class and implement the subtract operator on it. More or less the thing that we do when using Xojo.Core.Date.

For that, the first step is adding a new class to a Xojo Project (Insert > Class). Use the following data in the Inspector Panel:

  • Name: MyDateClass
  • Super: Date

Next, add to the class the same constant we defined in the previous section (kDay, kMonth and kYear). These will be used to define the keys in the resulting dictionary and to ease operations from the calling code too.

The most important thing is adding a new Method to our class with the following data:

  • Method Name: Operator_Subtract
  • Parameters: rightMostDate as Date
  • Return Type: Dictionary

The Operator_Subtract method instructs the Xojo compiler so you can use the minus sign (-) between two instances of the class. Type these lines of code for the method:

Dim difference As Double = self.TotalSeconds - rightMostDate.TotalSeconds

Dim days As Integer
Dim years As Integer
Dim months As Integer

Dim dateResult As New date

dateResult.TotalSeconds = difference

days = dateResult.Day - 1
years = dateResult.Year - 1904
months = dateResult.Month - 1

Dim result As New Dictionary

result.Value(kDay) = days
result.Value(kYear) = years
result.Value(kMonth) = months

Return result

Then in the Open Event of a Window, or the App object, check the class with the following code:

Dim d2 As New MyDateClass(2017,10,24)
Dim d3 As New MyDateClass(2017,11,30)

interval = d3 - d2

MsgBox interval.Value(kYear) + " " + interval.Value(kMonth) + " " + interval.value(kDay)

Javier Rodri­guez has been the Xojo Spanish Evangelist since 2008, he’s also a Developer, Consultant and Trainer who has be using Xojo since 1998. He manages AprendeXojo.com and is the developer behind the GuancheMOS plug-in for Xojo Developers and the Snippery app, among others.

*Read this post in Spanish

Published in Learning