Form Box Autofilling With Date And Number?

Hi all, and apologies for starting a fresh thread about this, but I can't find exactly what I'm trying to achieve via the search function, although I'm hoping it's just something simple I'm missing.

I have a small database that allows the user to click on a button to go an Add New Records form, which they then fill out. At the moment, the record number gets autofilled and they start entering details. However, one of the details they must enter is a unique record number, consisting of the month, the year, and a number between 01 and 99 (so the first new record created today, for example, would be record 090601, the second 090602, etc. The first next month would be 100601, and so on).
At the moment, I have to rely on people to manually enter this number, which requires them to view the current records first to ascertain whether there have been any previous records created that day (to prevent duplications).

What I'd really like is to have the this records box autofill when entering the form with the current month, current year, and then the number of the record for that day. However, it should automatically look at the last record created, and, if there's a match on the month and year, add one to the last 2 digits to ensure a new, unique record. So, for example, I have three records this month, listed as numbers 090601, 090602, and 090603 respectively. I go to create a new record, the box should autofill with the number 090604 and create a new record accordingly.

How do I acheive this?