Field to calculate the number of records created before current record?

Hi all, I need to create a field that will calculate the number of records that have been created before the current record with a date in the same year. I can't use record ID# because those numbers keep climbing regardless of year, and the way our numbering system works, it starts at 1 again in a new year (formatted Job #19001, for example). 

So I'm thinking the best way to accomplish this is with a field that displays the number of records created previously with a date in the same year as the current record, and then + 1 to whatever that number is.

Any ideas for the formula for this field?