Help with numeric formula field

I am trying to give each record entry a unique numeric value that conforms to the following convention: 
(Year - Month - Project number for the month)

As an example, for our 15th project this month, we would have the ID: 20-04-015. The project number resets to 0 at the beginning of each month, so in May the first project would be 20-05-000. The dashes aren't important here, I just included them for clarity -- I want this to be a numeric field. 

I thought I could do this by making a field of type Formula - Numeric where I get the last field entry value and increment, but it doesn't look like I can actually pull the last field value. 

Can someone please help me define a formula that will do this?

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Cypress Beach
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