DateTime. This VB.NET Structure represents a point in time. It stores times, like 12 AM, and dates like yesterday. With TimeSpan we capture time differences.

TimeSpan

Methods are used to compute relative times such as yesterday and tomorrow. DateTime can be formatted—this changes it to a String.

DateTime Format

New instance. A DateTime instance can be created with the New operator. There are many overloads. But the overload shown here accepts 3 arguments.

Module Module1
    Sub Main()
        ' Part 1: create new instance.
        Dim value As DateTime = New DateTime(2022, 2, 1)
        Console.WriteLine(value)

        ' Part 2: test the year.
        If value.Year = 2022 Then
            Console.WriteLine("Year is 2022")
        End If
    End Sub
End Module
2/1/2022 12:00:00 AM
Year is 2022

Yesterday. To get yesterday, we can access the Today property and then add negative one days to it. This is equivalent to subtracting one day from the DateTime instance.

Module Module1
    Sub Main()
        ' Write the today value.
        Console.WriteLine("Today: {0}", DateTime.Today)
        ' Subtract one day.
        Dim yesterday As DateTime = DateTime.Today.AddDays(-1)
        ' Write the yesterday value.
        Console.WriteLine("Yesterday: {0}", yesterday)
    End Sub
End Module
Today: 1/17/2010 12:00:00 AM
Yesterday: 1/16/2010 12:00:00 AM

Tomorrow. This is a good day to schedule less pleasurable tasks. To compute tomorrow, we add one day with Add Days() on the value returned by the Today property.

Module Module1
    Sub Main()
        ' Write Today.
        Console.WriteLine("Today: {0}", DateTime.Today)
        ' Add one to Today to get tomorrow.
        Dim tomorrow As DateTime = DateTime.Today.AddDays(1)
        ' Write.
        Console.WriteLine("Tomorrow: {0}", tomorrow)
    End Sub
End Module
Today: 1/17/2010 12:00:00 AM
Tomorrow: 1/18/2010 12:00:00 AM

First day. Getting the first day in a year is useful. We defines two new methods to get the first day in a year. When no argument is specified, the current year (for Today) is used.

Module Module1
    Sub Main()
        ' Write first day of current year.
        Console.WriteLine("First day: {0}", FirstDayOfYear)
        ' Write first day of 1999.
        Dim y As New DateTime(1999, 6, 1)
        Console.WriteLine("First day of 1999: {0}", FirstDayOfYear(y))
    End Sub

    ''' <summary>
    ''' Get first day of the current year.
    ''' </summary>
    Private Function FirstDayOfYear() As DateTime
        Return FirstDayOfYear(DateTime.Today)
    End Function

    ''' <summary>
    ''' Get first day of the specified year.
    ''' </summary>
    Private Function FirstDayOfYear(ByVal y As DateTime) As DateTime
        Return New DateTime(y.Year, 1, 1)
    End Function
End Module
First day: 1/1/2010 12:00:00 AM
First day of 1999: 1/1/1999 12:00:00 AM

Last day. Here we compute the last day in a year. This may help with year-based record keeping. The method gets the first day of the next year, and then subtracts one day.

Module Module1
    Sub Main()
        ' Write current last day.
        Console.WriteLine("Last day: {0}", LastDayOfYear)
        ' Write last day of 1999.
        Dim d As New DateTime(1999, 6, 1)
        Console.WriteLine("Last day of 1999: {0}", LastDayOfYear(d))
    End Sub

    ''' <summary>
    ''' Get last day of the current year.
    ''' </summary>
    Private Function LastDayOfYear() As DateTime
        Return LastDayOfYear(DateTime.Today)
    End Function

    ''' <summary>
    ''' Get last day of the specified year.
    ''' </summary>
    Private Function LastDayOfYear(ByVal d As DateTime) As DateTime
        Dim time As New DateTime((d.Year + 1), 1, 1)
        Return time.AddDays(-1)
    End Function
End Module
Last day: 12/31/2010 12:00:00 AM
Last day of 1999: 12/31/1999 12:00:00 AM

DaysInMonth. Months contain different numbers of days. With the shared DaysInMonth Function, we get day-counts for a month in a specific year. The 2 means February.

Module Module1
    Sub Main()
        ' There are 28 days in February.
        Dim count As Integer = DateTime.DaysInMonth(2014, 2)
        Console.WriteLine(count)

        ' Count days in March of 2014.
        Dim count2 As Integer = DateTime.DaysInMonth(2014, 3)
        Console.WriteLine(count2)
    End Sub
End Module
28
31

Get DayOfWeek. Every day in the future of our universe will have a day of week—like Monday or Tuesday. If you have a special day you need to test for, use the DayOfWeek property.

Module Module1
    Sub Main()
        ' Get current day of the week.
        Dim day As DayOfWeek = DateTime.Today.DayOfWeek

        ' Write the current day.
        Console.WriteLine("TODAY IS {0}", day)
    End Sub
End Module
TODAY IS Wednesday

DateString. This property, only offered in VB.NET, returns the current date in String representation. Usually, DateTime-based methods are preferable—they are more standard.

Module Module1
    Sub Main()
        Console.WriteLine(DateString)
    End Sub
End Module
05-18-2011

Date. This is another VB.NET special type. It aliases the DateTime type and has all the same methods. We can use Date interchangeably with DateTime.

Module Module1
    Sub Main()
        ' The Date type is the same as the DateTime type.
        Dim d As Date = New Date(2014, 10, 6)
        Dim d2 As DateTime = New DateTime(2014, 10, 6)

        If d = d2 Then
            Console.WriteLine("Equal dates")
        End If
    End Sub
End Module
Equal dates

DateTime.MinValue. Suppose we have a DateTime that we are not ready to assign to an actual value. We can use a special constant like MinValue to mean the default, zero value.

Module Module1

    Sub Main()
        Dim value As DateTime = DateTime.MinValue
        Console.WriteLine("MINVALUE: {0}", value)

        ' Test for MinValue.
        If value = DateTime.MinValue Then
            Console.WriteLine("MIN VALUE IS SET")
        End If
    End Sub

End Module
MINVALUE: 1/1/0001 12:00:00 AM
MIN VALUE IS SET

Error, un-representable. We cannot create a DateTime that cannot possibly exist. Here we try to use a month of 20 in the constructor, but get an "un-representable" error.

Module Module1
    Sub Main()
        ' Cannot have a month of 20, no such month exists.
        Dim value As DateTime = New DateTime(2000, 20, 1)
    End Sub
End Module
Unhandled Exception: System.ArgumentOutOfRangeException:
Year, Month, and Day parameters describe an un-representable DateTime.
   at System.DateTime.DateToTicks(Int32 year, Int32 month, Int32 day)
   at System.DateTime..ctor(Int32 year, Int32 month, Int32 day)...

Nothing. We cannot assign a DateTime to Nothing—we must use a special value like MinValue to mean "no date." But we can also use a Nullable DateTime.

Nullable

Module Module1

    Sub Main()
        ' Use nullable DateTime.
        Dim test As DateTime? = New DateTime(2000, 1, 1)
        Console.WriteLine("GETVALUEORDEFAULT: {0}", test.GetValueOrDefault())

        ' The nullable DateTime can be Nothing.
        test = Nothing
        Console.WriteLine("ISNOTHING: {0}", IsNothing(test))
    End Sub

End Module
GETVALUEORDEFAULT: 1/1/2000 12:00:00 AM
ISNOTHING: True

Summary. We used the DateTime type in the VB.NET language in some specific, useful ways. The examples provide a glimpse into the DateTime type but not a comprehensive view.