E-mail; Birthday reminder, Week difference

The issue you have here is that the spreadsheet needs to be open in order to check if there are any birthdays in the next 7 days (but I'm guessing you already know that and thats why none tackled the exact problem).

Its straightforward to do what you want as long as someone opens the spreadsheet at least once per week. Sometimes the obvious solution is the best ie stick a reminder in someones calendar to open the spreadsheet every monday morning.

Alternatively, you could change the way you're thinking about this problem and have the code insert a calendar reminder for 7 days prior to the employees birthday into a person(s) calendar(s) at the point of entering the new employees details.

As it would be doing that at the point of creating a new record then there wouldnt be any need to open the sheet at any time. make it a recurring appointment and it'll reappear every year on cue.