Bdtyktl

I have a list with dates (list 1). These dates stand for meetings.

To each of these meetings I assign a person, but sometimes the chosen person has already another appointment.

So I need to know, when I choose a person, that this person has not already another appointment (the appointments of different peoples will be saved in a separated excel list "list 2")!

When I'm right there has to be a formula that makes a dates comparison (between "list1" and "list2") and if the chosen person has an appointment on that specific date it should colour the column in "list1" where I want to enter the name of the person, red (as alert)!

Anyone has an idea?

What you need is called conditional formatting

To achieve what you need:
- select the column you want to format
- in the ribbon in home tab select "conditional formatting" - "new rule" - "use a formula to determine ..."
- enter formula
=COUNTIFS(Sheet2!$A:$A,A1,Sheet2!$B:$B,B1)>0
- click "format" - fill, select red and press ok twice

I hope I understand your requirement correctly. I have two lists of the same dates. I used 1 Feb till 28 Feb, and for simplicity, I only worked with the date, no time slots. But you could scale the functionality beyond just the date. Firstly, working Excel and dates, you first have to ensure that both lists are seen by Excel as the same data type. It might look like a date to you but if it's not in the right format Excel will see it as text (string).

Ok, So, the list on the right I populated with dates and a name next to each. This is the reference list. On the left, I have the same dates, but we're going to fill in the names to see if that person is available for that timeslot. I have a 3rd cell with the lookup formula in, and this will report the error if the user is busy.
The formula I used in column C (the warning Column) is as follows:

=IF(B2=VLOOKUP(A2;K2:L29;2;FALSE);"USER ALREADY BUSY";"")

The above is a copy of the formula from C2. This does a VLOOKUP on the date in B2, and compares it to the list (K2 to L29), and finds the name entered next to it. If it matches the name, it prints "User already busy", else we print nothing. I've attached a Screenshot for a better explanation.

Kindly note, I'm using a Mac and by default, my function separation character is a semi-colon, i.e. =VLOOKUP(A2;K2:L29;2;FALSE) and on Windows by default this will be a comma. i.e. =VLOOKUP(A2,K2:L29,2,FALSE). This is a regional setting on your PC. If your function separation character is a comma, use the following formula:

=IF(B2=VLOOKUP(A2,K2:L29,2,FALSE),"USER ALREADY BUSY","")