restarting filename iteration - vb.net

Question

I wanted to restart the iteration when the system clock hits 00:00 or 12:00 MN. I got the iteration code from the answer of this (below) link, and it works perfectly.
Public Sub GetLastNumber(ByVal filePath As String)
Dim lastFileNo As Integer = 1
Dim files() As String = Directory.GetFiles(filePath, "*.txt")
For Each file As String In files
file = Path.GetFileNameWithoutExtension(file)
Dim numbers As MatchCollection = Regex.Matches(file, "(?<num>[\d]+)")
For Each number In numbers
number = CInt(number.ToString())
If number > 0 And number < 1000 And number > lastFileNo Then lastFileNo = number
Next
lastnumber.Text = number
Next
End Sub
I stumbled something that uses a Timer like this one below, but it's giving me an error saying conversion fail "AM" as String to a Date type.
Public Sub DoStuff(ByVal obj As Object)
MessageBox.Show("It's already time", "TIME!", MessageBoxButtons.OK, MessageBoxIcon.Information)
End Sub
Public Sub testT()
Dim tcb As TimerCallback = AddressOf DoStuff
Dim t As Timer
Dim execTime As TimeSpan
Dim dtNow As DateTime = DateTime.Now
Dim hc As Integer = 12
Dim mc As Integer = 0
If TimeOfDay.ToString("tt").Contains("AM") And hc = 12 Then
hc = 0
ElseIf TimeOfDay.ToString("tt").Contains("PM") Then
hc = 12 + (12 - hc)
If hc = 24 Then
hc = 0
End If
End If
Dim dtCandidate As DateTime = New DateTime(dtNow.Year, dtNow.Month, dtNow.Day, hc, mc, 0)
If dtCandidate < dtNow Then
dtCandidate.AddDays(1)
End If
execTime = dtNow.Subtract(dtCandidate)
resultBox.Text = execTime.ToString
t = New Timer(tcb, Nothing, execTime, TimeSpan.Zero)
End Sub
Public Sub realTime_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles realTime.Tick
TimeNow.Text = TimeOfDay.ToString("HH:mm:ss")
testT()
End Sub
Conversion failure was remedied by using TimeOfDay.ToString("tt").Contains("AM/PM"). Un-representable DateTime error was remedied by correcting the ElseIf statements. Since there's no more error, I tried to put the testT function inside a Timer firing at 1000 ms. After system clock hit midnight(00:00), the message box of the DoStuff function showed every second after midnight. How can this be stopped but can still show up the next time the clock hits midnight?
Can somebody help me out?

Answer 1

The code you linked to for the timer is very bad because it is trying to use strings in the manipulation of DateTimes - they are very different things.
I created a new Windows Forms application with only a Label named "TimeNow":
Public Class Form1
Friend WithEvents realTime As Windows.Forms.Timer
Private lastAlarmTime As DateTime
Private alarmTimes As List(Of DateTime)
Private displayTime As String
Public Sub DoStuff()
MessageBox.Show("It's already time", "TIME!", MessageBoxButtons.OK, MessageBoxIcon.Information)
End Sub
Public Sub CheckForAnAlarmTime()
Dim dtNow As DateTime = DateTime.Now()
For Each tt In alarmTimes
' the timer interrupt handler is not necessarily called at exact times. Allow for that.
If tt > lastAlarmTime AndAlso tt < dtNow Then
lastAlarmTime = dtNow
DoStuff()
SetAlarmTimes()
Exit For
End If
Next
End Sub
Public Sub realTime_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles realTime.Tick
Dim candidateDisplayTime As String = DateTime.Now.ToString("HH:mm:ss")
' only update the UI if necessary
If candidateDisplayTime <> displayTime Then
displayTime = candidateDisplayTime
TimeNow.Text = displayTime
End If
CheckForAnAlarmTime()
End Sub
Private Sub SetAlarmTimes()
Dim dtNow As DateTime = DateTime.Now()
alarmTimes = New List(Of DateTime)
alarmTimes.Add(New DateTime(dtNow.Year, dtNow.Month, dtNow.Day, 12, 0, 0))
' Recommendation: do not use exactly midnight without extensive testing, i.e. test over day rollover, month rollover, and year rollover.
' With less testing, use a few milliseconds less than midnight.
alarmTimes.Add(New DateTime(dtNow.Year, dtNow.Month, dtNow.Day, 0, 0, 0).AddMilliseconds(-50))
End Sub
Private Sub SetUpAlarmsTimer()
SetAlarmTimes()
lastAlarmTime = DateTime.Now()
realTime_Tick(Me, EventArgs.Empty)
realTime = New Windows.Forms.Timer()
realTime.Interval = 200 ' 200ms will update it more accurately w.r.t. visual appearance
AddHandler realTime.Tick, AddressOf realTime_Tick
realTime.Start()
End Sub
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
SetUpAlarmsTimer()
End Sub
End Class
Change the alarmTimes to whatever you need to check that the alarm is raised only once per alarmTimes item.
I am not willing to wait until midnight to check if alarmTimes.Add(New DateTime(dtNow.Year, dtNow.Month, dtNow.Day, 0, 0, 0) without the .AddMilliseconds(-50) will work as required, or until the end of the month or year to be absolutely sure. And please don't forget about testing around the end of February when it is a leap year.
The reason for checking against the lastAlarmTime is that it is not certain when a timer event will be raised: for a timer set to tick at 1000ms, you might get two events inside one real second, or none inside one real second. Approximately.
EDIT: You might also want to work in UTC to avoid hassle with daylight savings time changes.