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 the video lesson: Expert Skills Lesson 3-24 Use a VLOOKUP function for an inexact lookup.
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:
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