pepper = sha256(global_randnum + user_start_dt + username + global_randnum) the above creates a per user pepper, and largely obviates the need for pepper rotat

pepper = sha256(global_randnum + user_start_dt + username + global_randnum) the above creates a per user pepper, and largely obviates the need for pepper rotation. if only password db is stolen, all users are impervious to dictionary style attacks. if only codebase is stolen, then they only know how to calculate each pepper per user. if both are stolen, you are in no worse shape than just using a salted bcbrypt, scrypt, or PBKDF2.