VBA ontime cancel scheduling

I have written a macro that runs at 15:30pm every workday when a workbook is first opened. When the workbook is closed it tries to open itself the next time the macro is scheduled to run. I have tried to turn the scheduler to false and am getting an error. Code below. Has anyone any ideas why this isn't working?

Private Sub Workbook_Open()
    Application.OnTime TimeValue("15:30:00"), "开发者_运维技巧;MacroTimeTest"
End Sub

public dtime as date

Sub MacroTimeTest()

    dtime = (Format(Application.Evaluate("workday(today(), 1)"), "DD/MM/YY") & " " & TimeValue("15:30:00"))

    'other code has been deleted doesn't affect dtime variable 
End Sub

Private Sub Workbook_BeforeClose(Cancel As Boolean)

    'I have tried replacing false with 0 etc but it didn't make a difference
    Application.OnTime earliesttime:=dtime, procedure:="MacroTimeTest", schedule:=False

End Sub

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I think that you should keep a reference to the time so that you can cancel the action. You can only cancel an action if it hasn't already executed.

In ThisWorkbook enter the following to run the macro at 15:59 until the sheet is closed

Option Explicit

Private Sub Workbook_BeforeClose(Cancel As Boolean)

    On Error GoTo CouldNotCancel

    Application.OnTime dTime, "MacroTimeTest", , False
    Debug.Print "Cancelled task to run at " & dTime

    Debug.Print "Workbook close"

    Exit Sub


CouldNotCancel:
    Debug.Print "No task to cancel"

End Sub 

Private Sub Workbook_Open()
    Debug.Print "Workbook open"

    dTime = TimeValue("15:59:00")

    Debug.Print "Next run time " & dTime
    Application.OnTime dTime, "MacroTimeTest"

End Sub

Then add your macro to a Module

Option Explicit
Public dTime As Date
Public Sub MacroTimeTest()

    'schedule next run
    dTime = TimeValue("15:59:00")

    'schedule next run
    Debug.Print "Scheduling next run at " & dTime

    Application.OnTime dTime, "MacroTimeTest"

    Debug.Print "Running macro"

End Sub

This way the same value of dTime will be used to cancel the scheduled task as was used to create it.

If no further task has been scheduled i.e. by an error in MacroTimeTest then the Workbook close event will handle the error.

To see the debug output look at the immediate window in the VBA Editor (Ctrl+G)