PostgreSQL: Issue Declaring Variables in Query

I'm trying to declare a variable for later use in a PostgreSQL query. I've done this often in TSQL, but I wasn't sure about the syntax. The stuff I've seen online all points to something like this: declare timestamp_utc timestamp := current_timestamp; select start_date; when I run the above query, I get an error message: I'm sure this is simple, but I just can't find the answer online. Any help you could provide would be greatly appreciated.