Public Declare Sub GetLocalTime Lib "kernel32" (lpSystemTime As SYSTEMTIME)

Public Type SYSTEMTIME
        wYear As Integer
        wMonth As Integer
        wDayOfWeek As Integer
        wDay As Integer
        wHour As Integer
        wMinute As Integer
        wSecond As Integer
        wMilliseconds As Integer
End Type

Dim t As String, delta As Long, st As SYSTEMTIME
t = Range("A1").Text
If Not IsDate(t) Then MsgBox "А1 не содержит корректное время": Exit Sub
Debug.Print DateDiff("s", Time, Fix(CDate(t)) - CDate(t))
delta = DateDiff("s", Time, CDate(t) - Fix(CDate(t)))

Do
  Call GetLocalTime(st)
  Range("A2") = Format$(TimeSerial(st.wHour, st.wMinute, st.wSecond + delta), "hh:nn:ss") + "." + CStr(st.wMilliseconds)
  DoEvents
Loop