#1 App crashes on second click on step

[BatZzZz]

On second click of the "step" button, the app crashes.
Windows 7 64-bit. dot-Net FW installed (up to 4.0)
************** Exception Text **************
System.OverflowException: Arithmetic operation resulted in an overflow.
at BPM_Calc.calcPoint.getMillisecs(DateTime dt)
at BPM_Calc.calcPoint.get_timeBetween()
at BPM_Calc.calcPoint.ToString()
at System.Convert.ToString(Object value, IFormatProvider provider)
at System.Windows.Forms.ListControl.GetItemText(Object item)
at System.Windows.Forms.ListBox.NativeAdd(Object item)
at System.Windows.Forms.ListBox.ObjectCollection.AddInternal(Object item)
at System.Windows.Forms.ListBox.ObjectCollection.Add(Object item)
at BPM_Calc.main.btnStart_Click(Object sender, EventArgs e)
at System.Windows.Forms.Control.OnClick(EventArgs e)
at System.Windows.Forms.Button.OnClick(EventArgs e)
at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
at System.Windows.Forms.Control.WndProc(Message& m)
at System.Windows.Forms.ButtonBase.WndProc(Message& m)
at System.Windows.Forms.Button.WndProc(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

************** Loaded Assemblies **************
mscorlib
Assembly Version: 2.0.0.0
Win32 Version: 2.0.50727.4952 (win7RTMGDR.050727-4900)
CodeBase: file:///C:/Windows/Microsoft.NET/Framework/v2.0.50727/mscorlib.dll
----------------------------------------
BPM Calc
Assembly Version: 1.0.0.0
Win32 Version: 1.0.0.0
CodeBase: file:///C:/Users/Itai/Downloads/BPM%20Calc/BPM%20Calc/BPM%20Calc.exe
----------------------------------------
Microsoft.VisualBasic
Assembly Version: 8.0.0.0
Win32 Version: 8.0.50727.4927 (NetFXspW7.050727-4900)
CodeBase: file:///C:/Windows/assembly/GAC_MSIL/Microsoft.VisualBasic/8.0.0.0__b03f5f7f11d50a3a/Microsoft.VisualBasic.dll
----------------------------------------
System
Assembly Version: 2.0.0.0
Win32 Version: 2.0.50727.4927 (NetFXspW7.050727-4900)
CodeBase: file:///C:/Windows/assembly/GAC_MSIL/System/2.0.0.0__b77a5c561934e089/System.dll
----------------------------------------
System.Windows.Forms
Assembly Version: 2.0.0.0
Win32 Version: 2.0.50727.4927 (NetFXspW7.050727-4900)
CodeBase: file:///C:/Windows/assembly/GAC_MSIL/System.Windows.Forms/2.0.0.0__b77a5c561934e089/System.Windows.Forms.dll
----------------------------------------
System.Drawing
Assembly Version: 2.0.0.0
Win32 Version: 2.0.50727.4927 (NetFXspW7.050727-4900)
CodeBase: file:///C:/Windows/assembly/GAC_MSIL/System.Drawing/2.0.0.0__b03f5f7f11d50a3a/System.Drawing.dll
----------------------------------------
System.Runtime.Remoting
Assembly Version: 2.0.0.0
Win32 Version: 2.0.50727.4927 (NetFXspW7.050727-4900)
CodeBase: file:///C:/Windows/assembly/GAC_MSIL/System.Runtime.Remoting/2.0.0.0__b77a5c561934e089/System.Runtime.Remoting.dll
----------------------------------------

************** JIT Debugging **************
To enable just-in-time (JIT) debugging, the .config file for this
application or computer (machine.config) must have the
jitDebugging value set in the system.windows.forms section.
The application must also be compiled with debugging
enabled.

For example:

<configuration>
<system.windows.forms jitDebugging="true" />
</configuration>