Complex Forecasting Formula Needed?

Hi all,

Thanks for reading and helping with me this problem. I am in need to write a formula that can distinguish two different logical situations and output a formula based on which one it is.
The first being any center that has greater than 24 months of data. The other being any center with less than 24 months of data.
Please see the attachment for the actual file and description.

If I left anything out please ask me to clarify.
Thanks

Scottie