Таймер заходит после первой минуты за 60 секунд

        Dim Hrs As Long
        Dim Mins As Long
        Dim Secs As Long
        Dim Secunds As Integer
        Secs = DateDiff(DateInterval.Second, dStart, DateTime.Now)
        Secunds = Secs
        While (Secunds - 59) > 0
            Secunds = Secunds - 60
            Mins = Mins + 1
        End While
        While (Mins - 59) > 0
            Mins = Mins - 60
            Hrs = Hrs + 1
        End While
        Label15.Text = Format(Hrs, "00") & ":" & Format(Mins, "00") & ":" & Format(Secs, "00")
        If Mins = 5 Then
            Timer1.Stop()
            Dim i As Integer
            For i = 0 To 10
                Me.TabControl1.TabPages(i).Enabled = False
            Next i
        End If

Secunds = Secs
        While (Secunds - 59) > 0
            Secunds = Secunds - 60
            Mins = Mins + 1
        End While
        
 
        Label15.Text = Format(Hrs, "00") & ":" & Format(Mins, "00") & ":" & Format(Secs, "00")

Private startTime As DateTime
Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
    Timer1.Interval = 1000
    Label1.Text = ""
End Sub
Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
    Timer1.Start()
    startTime = Now
End Sub
Private Sub Timer1_Tick(sender As System.Object, e As System.EventArgs) Handles Timer1.Tick
    Dim dd As TimeSpan = Now - startTime
    Label1.Text = Format(dd.Hours, "00") & ":" & Format(dd.Minutes, "00") & ":" & Format(dd.Seconds, "00")
    If dd.Minutes = 5 Then
        startTime = Now
        '...
    End If
End Sub