Auto-generate part numbers list in one column

I am looking for help auto-generating a part numbering list in one column. I want it to trigger when you enter into the row below the previous part number.

An example part number would be 915010001.
The "9" is constant no matter what.
"15" signifies that it is the year 2015
"01" signifies that it is the month of January
"0001" means that it is the first part number of the month
Another example:
912111245
"9" is constant
It is made in the year 2012 (12)
It is made in the month of November (11)
It is the 1,245th part made that month

Attached is what the spreadsheet will look like.
Possibly a msgbox error handler would help because many people who do not know anything about VBA will be using this spreadsheet.
PartNumbering.jpg

I guess I jumped the gun on that one. I added the "Else:" and now it deletes the part number when I add anything in the same row.