Find the Monday of a Week, Based on "Date" Field - vb.net

Question

This is my issue, I want to return the Monday of a given week in date format.
Here I have my effort so far.
Code:
Dim Diff
Dim cDate
Dim prday
prday = [table.datefield]
Diff = Cint(0 - (Weekday(prday, 2)))
cDate = DateAdd("d", Diff, prday)
Result = cDate
However, this rather unfortunately returns 24/12/1899. Not exactly what I want.. 2013 would be nice. Any tips for where I went wrong?
Note:
The specific date which is pulled out of the database table is done in a different application. This code is supposed to generate a date which will be shown on a label.

Answer 1

The only issue is how you want to treat Sunday dates.
'Sunday as first day of week
Dim someDate As DateTime = DateTime.Parse("Apr 28, 2013")
'calculate monday
'note: for Sunday this returns next Monday, i.e Apr 28, 2013 returns Apr 29th
Dim monDate As DateTime = someDate.AddDays(DayOfWeek.Monday - someDate.DayOfWeek)
'Monday as first day of week
Dim someDate As DateTime = DateTime.Parse("Apr 28, 2013")
'calculate monday
If someDate.DayOfWeek = DayOfWeek.Sunday Then someDate = someDate.AddDays(-1)
Dim monDate As DateTime = someDate.AddDays(DayOfWeek.Monday - someDate.DayOfWeek)

Answer 2

Define an extension method
Imports System.Runtime.CompilerServices
Module DateExtensions
<Extension()>
Public Function GetMonday(ByVal dt As DateTime) as DateTime
While dt.DayOfWeek <> DayOfWeek.Monday
dt = dt.AddDays(-1)
End While
return dt
End Function
End Module
And call it
prday = prday.GetMonday()
Or we could make it generic
Public Function GetPreviousDayOfWeek(ByVal dt As DateTime, prevDay As DayOfWeek) as DateTime
While dt.DayOfWeek <> prevDay
dt = dt.AddDays(-1)
End While
return dt
End Function
As pointed out by #dbasnett a math is faster than a loop and we could handle the situation when Sunday is the first day of week. So a better approach should be
Public Function GetMonday(ByVal dt As DateTime) as DateTime
Dim diff = Thread.CurrentThread.CurrentCulture.DateTimeFormat.FirstDayOfWeek - DayOfWeek.Monday
dt = dt.AddDays(diff)
return dt.AddDays(DayOfWeek.Monday - dt.DayOfWeek)
End Function