将属性设置为另一个属性的值

I'm pretty new to this and my issue is very particular. I've googled a lot and haven't been able to find anything on this. I have two properties in a database, StartDate and StartMonth. The StartDate is of type DateTime and the StartMonth is of string type. (I know it's dumb to save both of these into the database but this isn't something I can change because I don't have the authority to do so.) My question is how in the DB Model can I set the StartMonth to equal the Month in the StartDate (since it is the same)? Rather, what is the proper way to do this? Any help is really appreciated!I'm pretty new to this and my issue is very par