Auto Run Macro Every X Minutes Or Time Specified In Cell

The below seems to work but I'm wondering if there might be a better way. I'm trying to keep an ongoing up-to- date and accurate time. my code is as follows:

Private Sub Workbook_Open() ' placed inside thisworkbook
Call TimeUp
End Sub

Sub UpdateTime() ' placed in module
If Range("A4") = TimeValue("00:00:00") Then
Application.OnTime Now + TimeValue("00:01:00"), "TimeUp"
Else
UpdateTime2
End If
End Sub

Sub TimeUp()
[a1] = Time
UpdateTime
End Sub

Sub UpdateTime2()
[a1] = Time
Application.OnTime Now + Range("A4").Value, "TimeUp"
End Sub

Does anyone know if there is any way to improve the Code or formulas within the Cells?