Need help with excel referencing to outlook
 

I have an excel spreadsheet in which i'm using as an invoice template. One of
the fields of input is a schedule date with time and date. I am also using
Outlook's calender as my appointment tracker. What I'm trying to do is write
in a formula or a small macro that would let me type in the time and date on
the invoice in excel and then have it cross reference with my calender and if
there is a problem with that i want some kind of notification, either thru
the program or user defined. any suggestions?