yyyymmddhhMMss + ミリ秒

'現在の時刻をyyyymmddHHMMSS+ミリ秒で取得
Function getNow() As String
    getNow = Format(Now, "yyyymmddHHMMSS") & getMSec()
End Function

'ミリ秒を取得
Function getMSec() As String
    Dim dblTimer As Double
    Dim s_return As String
    dblTimer = CDbl(Timer)
    s_return = Format(Fix((dblTimer - Fix(dblTimer)) * 1000), "000")
    getMSec = s_return
End Function

※他言語ではyyyymmddhhMMssとyyyymmddHHMMSSで取れる値が違うので注意。