I have taken a look and there were two issues that were causing problems.
Firstly, your MATCH function was using inexact matching, so it wasn't always finding the correct column for each date. Setting its last argument to 0 tells it to use exact matching and solves this issue.
You can see more about exact versus inexact matching in our Expert Skills Books and E-books.
This seems to solve the first problem you noticed.
The second problem was that the formulas were returning #N/A after changing the date. The reason for this is my fault - in my version of the formulas I changed the TEXT function to this:
This works perfectly for dates with two-digit days, but your RSVPTracker worksheet only has single digits for the days, meaning my formula was trying to match 4/04/16 with 4/4/16 and failing to find a match.
The correct formula looks like this:
Making this change seems to fix the formula so that it always matches the dates correctly.
I have attached another copy of the workbook with the changes applied:
Jonathan is part of the professional team who answer Excel-related questions posted on the ExcelCentral.com forums.
Jonathan also tests our courses prior to publication and has worked on all of our ten world bestselling Excel books for Excel 2007, Excel 2010, Excel 2013, Excel 2016 for Windows and Excel 2016 for Apple Mac. Jonathan has also worked on over 850 video lessons for or video courses covering Excel 2007, Excel 2010 and Excel 2013.
As well as extensive Excel knowledge, Jonathan has worked in the IT world for over thirteen years as a programmer, database designer and analyst for some of the world's largest companies.