关于日期转换计算的有关问题

关于日期转换计算的问题
我有一个日期型变量，该值作为参数传入，我还定义了个日期型变量，我想再接入该变量后，将该日期设为两个时间，1个是起始时间，另1个是结束时间

如“2009/01/20”(该值为日期型) 传入后， 

Dim dtstart as date = '2009-01-01'
Dim dtfinish as date = '2009-01-31' (如1月最后1天为31；2月为29或28)

请问应如何实现呢，谢谢！


[解决办法]

VB.NET code

    Private Function GetLastDate(ByVal istrYMD As String) As Date        Dim objDate As DateTime = DateTime.Parse(istrYMD)        Dim intYear As Integer = objDate.Year        Dim intMonth As Integer = objDate.Month        Dim intDay As Integer = DateTime.DaysInMonth(intYear, intMonth)        Return New DateTime(intYear, intMonth, intDay)    End Function    Private Function GetFirstDate(ByVal istrYMD As String) As Date        Dim objDate As DateTime = DateTime.Parse(istrYMD)        Dim intYear As Integer = objDate.Year        Dim intMonth As Integer = objDate.Month        Dim intDay As Integer = 1        Return New DateTime(intYear, intMonth, intDay)    End Function
[解决办法]
日期类功能很全了哦，


Module Module1

   Sub Main()
       Dim start As DateTime
       Dim finish As DateTime
       GetDate(DateTime.Now, start, finish)
       Console.WriteLine(start.ToString("yyyy-MM-dd"))
       Console.WriteLine(finish.ToString("yyyy-MM-dd"))
   End Sub

   Sub GetDate(ByVal dt As DateTime, ByRef startDate As DateTime, ByRef finishDate As DateTime)
       startDate = New DateTime(dt.Year, dt.Month, 1)
       finishDate = New DateTime(dt.Year, dt.Month, DateTime.DaysInMonth(dt.Year, dt.Month))
   End Sub

End Module

引用上面的。