In Excel, Excel would have to be running first. You could build a VBA Event. There are some vulnerabilities to this running correctly if you run multiple sessions.
In Outlook(Desktop) may be better or similar. Though there is a move to the Online email services.
The best Reminder methods is the (probably) least known. Do a search on your Windows7 machine for "Task Scheduler". In Windows10, the Alarm Clock is better.
Here is a simple timer that pops up a Message Box at the end of the time you set.
Presently, the time is set to 10 seconds. You can edit that to any length time you require.
If SchedSave <> 0 Then
Application.OnTime SchedSave, "TimeOver", , False
SchedSave = Now + TimeValue("00:00:10") '<--- Ten seconds .. adjust time as required
Application.OnTime SchedSave, "TimeOver", , True
MsgBox "Your time is up !"
Paste the above code into a regular module. Paste a button on the sheet and attach it to the "StartTimer" macro.