use of Application.ontime

Hello,
This (see below) is an example of the use of Application.OnTime RunTime.
It is supposed the wait for 10 seconds and increment sheet1.cells(1,1).value by 15.
It does not. What am I missing?
Once it works with "00:00:10", I was hoping to replace "00:00:10" by a string that is the result of the concatenation of h:m:s that a user could input. Is this possible? Do I need to add chr$(34) at the beginning and the end? i.e. is aa going to work as is or aa=chr$(34)+aa+chr$(34)?

Please advise

Thank you


CODE ------------------------------START------------------------

Public Sub test_run_time()
Debug.Print Now
Call TestMacroAutoRun(0, 0, 10)
Debug.Print Now
End Sub

Sub TestMacroAutoRun(h, m, s)
Dim RunTime As Date
Dim aa As Date

hh = conv(h)
mm = conv(m)
ss = conv(s)
aa = hh + ":" + mm + ":" + ss
RunTime = Now + TimeValue("00:00:10")
Application.OnTime RunTime, "zipi"
Debug.Print aa
Application.OnTime RunTime, "zipi", , False
End Sub

Public Function conv(h) As String

If h < 10 Then
conv = "0" + LTrim(RTrim(Str$(h)))
Else
conv = LTrim(RTrim(Str$(h)))
End If
If h = 0 Then
conv = "00"
End If
End Function

Sub zipi()
Sheet1.Cells(1, 1).Value = Sheet1.Cells(1, 1).Value + 15
End Sub

CODE---------------------- STOP ---------------------------------