Macro for Message Popup

Hello,
I'm developing a register for a hostel, given two set of dates (check-in and check-out), i need to have a message popup whenever the stay is exceeded by 3 weeks.I would really appreciate if you could help me out with this one:

Please See DEMO File.

Please Login or Register  to view this content.

Except D2,E2,F2 are all columns with 54 rows, so when the user enters the check-in date and check-out date, D2, E2, F2 are computed and the macro should immediately alert the user if the above conditions are met.I'm really new to VBA, if you could code the macro for me that would be great.

Thanks
AV

Your post does not comply with Rule 3 of our Forum RULES. Use code tags around code. Posting code without them makes your code hard to read and difficult to be copied for testing. Highlight your code and click the # at the top of your post window. For more information about these and other tags, found here

How about in G2 and copy down,

=IF(C2-B2>21, "Stay cannot exceed 3 weeks", "")

Shg's formula or Conditional Formatting is the best option. There's no point using VBA when Excel has inbuilt features that work, the most obvious reason is that if macros aren't enabled then a VBA solution cannot work

If you're not using Data Validation on B & C already then you could consider using a Custom Validation Rule - see attached
(with additional test in D:F calculations)

(note: my question re: week calculation still holds - why is 1-Oct to 29-Oct 3 weeks 5 weekdays and 2 nights rather than 4 weeks ?)