I found a workaround for this. If you write a method in a module that just calls a method in your user form, then you can schedule the module method using Application.OnTime().
Kind of a kludge, but it'll do unless somebody has a better suggestion.
Here's an example:
''//Here's the code that goes in the user form
Dim nextTriggerTime As Date
Private Sub UserForm_Initialize()
ScheduleNextTrigger
End Sub
Private Sub UserForm_Terminate()
Application.OnTime nextTriggerTime, "modUserformTimer.OnTimer", Schedule:=False
End Sub
Private Sub ScheduleNextTrigger()
nextTriggerTime = Now + TimeValue("00:00:01")
Application.OnTime nextTriggerTime, "modUserformTimer.OnTimer"
End Sub
Public Sub OnTimer()
''//... Trigger whatever task you want here
''//Then schedule it to run again
ScheduleNextTrigger
End Sub
''// Now the code in the modUserformTimer module
Public Sub OnTimer()
MyUserForm.OnTimer
End Sub