Half-hour countdown timer issues

[uniden32]

Issue I'm having is that when the timer initially starts, everything is computed correctly, timer works great.

When the countdown timer hits zero, I have it restart from the beginning, but the timer usually starts off at 23-25 minutes, instead of 30 minutes.

Data sheet formulas:

D2 - Time (value is sent here in StartTimer .Value = Time)
D3 - =Hour(D2)
D4 - =Minute(D2)
D5 - =Second(D2)
E4 - =IF($D$4>=30,59-$D$4,29-$D$4)
E5 - =SUM(60-$D$5)

(The formulas in E4 and E5 are used to determine whether I'm counting down to the top of the our or down to the bottom of the hour)

Dim NextTick As Date

Sub StartTimer()
With ThisWorkbook.Sheets("Data").Range("D2")
    .NumberFormat = "hh:mm:ss"
    .Value = Time
End With
Call FormatTime
Call TickTock
End Sub

Private Sub FormatTime()
With ThisWorkbook.Sheets("Sheet1").Range("D4")
    .NumberFormat = "mm:ss"
    .Value = ThisWorkbook.Sheets("Data").Range("E3")
End With
End Sub

Private Sub TickTock()
With ThisWorkbook.Sheets("Sheet1").Range("D4")
    .NumberFormat = "mm:ss"
    .Value = ThisWorkbook.Sheets("Sheet1").Range("D4")
    .Value = .Value - (1 / 86400)
    If .Value <= (1 / 86400) Then
       Beep
       Call StopClock
       Call StartTimer
    End If
End With

NextTick = Now + TimeValue("00:00:01")
Application.OnTime NextTick, "TickTock"

End Sub

Sub StopClock()
On Error Resume Next
Application.OnTime earliesttime:=NextTick, procedure:="TickTock", schedule:=False
On Error GoTo 0
End Sub