Message box appear based on different criteria - excel

Question

I have some complicated Excel formatting to do, but I have no idea where to begin.
There are few criteria that need to be met:
When there are cell values in F3 and H3, the time at cell N4 should automatically be the time in N3 plus three hours.
If system time is exceeds the value in N4, and the value in F4 and H4 is still empty, it will trigger a message or format highlighting to inform user that the time is over.
If there is new bath being created in column B, the time in column N will stop adding the time and just left blank for user to key in the new time.

Answer 1

For 1. & 3., you'd simply use an if formula:
=IF(AND(F3<>"",H3<>"",B3=B4),N3+3/24,"")
This is assuming the value in N3 is a time value, otherwise it gets a bit trickier - but hardly impossible.
Your problem statement no. 2 is somewhat trickier, I don't think you can it without VBA, and the code isn't entirely straightforward. However, I'll give it a go. In a similar-ish workbook, I have a line which moves forward to indicate when different tasks needs to be completed. To have the "line" move, I color in a column of cells using conditional formatting, comparing the column of cells to a timestamp I have in Y29. The code I use to update the timestamp is the following, it updates the cells every minute:
In a standard module
Option Explicit
Public setDate As Date
Public Const nameToIgnore = "OAA0006Monitor"
Sub UpdateTime()
If currentUser() = nameToIgnore Then
Else
setDate = Now() + TimeValue("00:01:00")
Application.OnTime setDate, "TimeUp"
End If
End Sub
Sub TimeUp()
If currentUser() = nameToIgnore Then
Else
ThisWorkbook.Worksheets("ArbPlan 2011 (5)").Range("Y29") = Time
UpdateTime
End If
End Sub
Sub KillOnTime()
If currentUser() = nameToIgnore Then
Else
On Error Resume Next
Application.OnTime setDate, "TimeUp", , False
On Error GoTo 0
End If
End Sub
In the ThisWorkbook-module
Option Explicit
Private Sub Workbook_Activate()
UpdateTime
End Sub
Private Sub Workbook_Deactivate()
KillOnTime
End Sub
I don't think this should be very hard to convert to the purpose you want it for, instead of using conditional formatting, you can even have the code do all the formatting for you! :)
Anyway, I hope this gives you a good starting point, feel free to comment if you feel anything is unclear, or if you have trouble understanding any of the logic of the code.