There isn’t a built-in formula  function in Excel that will let you set  an alarm
on a cell. Here’s the best approach I have come up with since reading your
question:

1. Add a VBA module to your workbook. Paste the following custom
function into it:

Public Function CheckTime() As Boolean
If Application.Caller.Value <= Now() Then
CheckTime = True
Else
CheckTime = False
End If
End Function

2. Paste the following event code in the code window for ThisWorkbook:

Private Sub Workbook_SheetActivate(ByVal Sh As Object)
'Calc whenever any sheet in this workbook is activated
Application.Calculate
End Sub

Private Sub Workbook_SheetSelectionChange(ByVal Sh As Object, ByVal Target As
Range)
'Calc whenever a different cell is selected in this workbook
Application.Calculate
End Sub

3. Back in regular Excel, select one cell on the worksheet with a time
in it (for example, B9). Select Conditional Formatting from the Format menu.
The Conditional Formatting dialog appears. Select ‘Formula is” and enter the
following formula:

=AND(CheckTime()=TRUE,LEN(B9)>0)

Click the Format button. On the Patterns tab, click the color you want, then
OK. Click OK to end the Conditional Formatting dialog. Use Format Painter to
copy this cell’s formatting to every cell (or whole columns) where the times
will be stored.

Every time  the workbook is recalculated, the conditional formatting will call
CheckTime (for each cell with that formatting). If the time in a cell is
earlier or the same as Now(), CheckTime will return True, and the cell will be
colored red  (or whatever color you chose).

The workbook will recalculate whenever the workbook is activated, whenever a
different cell is selected in the workbook, or when you press F9. I set it up
this way because the OnTime event in Excel VBA is not reliable – it is too
easily disrupted.

I used the Now() function in my test workbook, so my cells  have both date &
time. If you have times only (no dates), the CheckTime function would have to be
modified. It would be easy to put a time-stamp button on your worksheet, which
would enter the current & future times as values. If you want help  with this, or
have trouble testing the other suggestions, let me know and I will try to help.